diff --git a/src/auth/userdb-dict.c b/src/auth/userdb-dict.c
index 6ccd198bdc..90ddb400a4 100644
--- a/src/auth/userdb-dict.c
+++ b/src/auth/userdb-dict.c
@@ -141,11 +141,15 @@ static int userdb_dict_iterate_deinit(struct userdb_iterate_context *_ctx)
 {
 	struct dict_userdb_iterate_context *ctx =
 		(struct dict_userdb_iterate_context *)_ctx;
+	const char *error;
 	int ret = _ctx->failed ? -1 : 0;
 
 	if (ctx->iter != NULL) {
-		if (dict_iterate_deinit(&ctx->iter) < 0)
+		if (dict_iterate_deinit(&ctx->iter, &error) < 0) {
+			i_error("dict_iterate(%s) failed: %s",
+				ctx->key_prefix, error);
 			ret = -1;
+		}
 	}
 	auth_request_unref(&ctx->ctx.auth_request);
 	i_free(ctx);
diff --git a/src/dict/dict-commands.c b/src/dict/dict-commands.c
index 43e0d93102..712a73e320 100644
--- a/src/dict/dict-commands.c
+++ b/src/dict/dict-commands.c
@@ -33,8 +33,12 @@ static void dict_connection_cmd_output_more(struct dict_connection_cmd *cmd);
 
 static void dict_connection_cmd_free(struct dict_connection_cmd *cmd)
 {
-	if (cmd->iter != NULL)
-		(void)dict_iterate_deinit(&cmd->iter);
+	const char *error;
+
+	if (cmd->iter != NULL) {
+		if (dict_iterate_deinit(&cmd->iter, &error) < 0)
+			i_error("dict_iterate() failed: %s", error);
+	}
 	i_free(cmd->reply);
 
 	if (dict_connection_unref(cmd->conn))
@@ -106,7 +110,7 @@ static int cmd_lookup(struct dict_connection_cmd *cmd, const char *line)
 static int cmd_iterate_flush(struct dict_connection_cmd *cmd)
 {
 	string_t *str;
-	const char *key, *value;
+	const char *key, *value, *error;
 
 	str = t_str_new(256);
 	o_stream_cork(cmd->conn->output);
@@ -138,8 +142,10 @@ static int cmd_iterate_flush(struct dict_connection_cmd *cmd)
 	}
 
 	str_truncate(str, 0);
-	if (dict_iterate_deinit(&cmd->iter) < 0)
-		str_append_c(str, DICT_PROTOCOL_REPLY_FAIL);
+	if (dict_iterate_deinit(&cmd->iter, &error) < 0) {
+		i_error("dict_iterate() failed: %s", error);
+		str_printfa(str, "%c%s", DICT_PROTOCOL_REPLY_FAIL, error);
+	}
 	str_append_c(str, '\n');
 	o_stream_uncork(cmd->conn->output);
 
diff --git a/src/doveadm/doveadm-dict.c b/src/doveadm/doveadm-dict.c
index e266ce9e21..29310568ac 100644
--- a/src/doveadm/doveadm-dict.c
+++ b/src/doveadm/doveadm-dict.c
@@ -186,7 +186,7 @@ static void cmd_dict_iter(int argc, char *argv[])
 	struct dict *dict;
 	struct dict_iterate_context *iter;
 	enum dict_iterate_flags iter_flags = 0;
-	const char *key, *value;
+	const char *key, *value, *error;
 
 	if (cmd_dict_init_full(&argc, &argv, 1, 0, cmd_dict_iter, &iter_flags, &dict) < 0)
 		return;
@@ -202,8 +202,8 @@ static void cmd_dict_iter(int argc, char *argv[])
 		if ((iter_flags & DICT_ITERATE_FLAG_NO_VALUE) == 0)
 			doveadm_print(value);
 	}
-	if (dict_iterate_deinit(&iter) < 0) {
-		i_error("dict_iterate_deinit(%s) failed", argv[0]);
+	if (dict_iterate_deinit(&iter, &error) < 0) {
+		i_error("dict_iterate_deinit(%s) failed: %s", argv[0], error);
 		doveadm_exit_code = EX_TEMPFAIL;
 	}
 	dict_deinit(&dict);
diff --git a/src/lib-dict-extra/dict-fs.c b/src/lib-dict-extra/dict-fs.c
index 91a40f92c8..82ea827308 100644
--- a/src/lib-dict-extra/dict-fs.c
+++ b/src/lib-dict-extra/dict-fs.c
@@ -21,7 +21,7 @@ struct fs_dict_iterate_context {
 	enum dict_iterate_flags flags;
 	pool_t value_pool;
 	struct fs_iter *fs_iter;
-	bool failed;
+	char *error;
 };
 
 static int
@@ -153,10 +153,13 @@ static bool fs_dict_iterate(struct dict_iterate_context *ctx,
 	const char *path, *error;
 	int ret;
 
+	if (iter->error != NULL)
+		return FALSE;
+
 	*key_r = fs_iter_next(iter->fs_iter);
 	if (*key_r == NULL) {
 		if (fs_iter_deinit(&iter->fs_iter) < 0) {
-			iter->failed = TRUE;
+			iter->error = i_strdup(fs_last_error(dict->fs));
 			return FALSE;
 		}
 		if (iter->paths[++iter->path_idx] == NULL)
@@ -173,8 +176,7 @@ static bool fs_dict_iterate(struct dict_iterate_context *ctx,
 	path = t_strconcat(iter->paths[iter->path_idx], *key_r, NULL);
 	if ((ret = fs_dict_lookup(ctx->dict, iter->value_pool, path, value_r, &error)) < 0) {
 		/* I/O error */
-		i_error("%s", error);
-		iter->failed = TRUE;
+		iter->error = i_strdup(error);
 		return FALSE;
 	} else if (ret == 0) {
 		/* file was just deleted, just skip to next one */
@@ -183,20 +185,24 @@ static bool fs_dict_iterate(struct dict_iterate_context *ctx,
 	return TRUE;
 }
 
-static int fs_dict_iterate_deinit(struct dict_iterate_context *ctx)
+static int fs_dict_iterate_deinit(struct dict_iterate_context *ctx,
+				  const char **error_r)
 {
 	struct fs_dict_iterate_context *iter =
 		(struct fs_dict_iterate_context *)ctx;
+	struct fs_dict *dict = (struct fs_dict *)ctx->dict;
 	int ret;
 
 	if (iter->fs_iter != NULL) {
-		if (fs_iter_deinit(&iter->fs_iter) < 0)
-			iter->failed = TRUE;
+		if (fs_iter_deinit(&iter->fs_iter) < 0 && iter->error == NULL)
+			iter->error = i_strdup(fs_last_error(dict->fs));
 	}
-	ret = iter->failed ? -1 : 0;
+	ret = iter->error != NULL ? -1 : 0;
+	*error_r = t_strdup(iter->error);
 
 	pool_unref(&iter->value_pool);
 	i_free(iter->paths);
+	i_free(iter->error);
 	i_free(iter);
 	return ret;
 }
diff --git a/src/lib-dict/dict-client.c b/src/lib-dict/dict-client.c
index efdf32d1a4..40378c6016 100644
--- a/src/lib-dict/dict-client.c
+++ b/src/lib-dict/dict-client.c
@@ -55,9 +55,9 @@ struct client_dict {
 
 struct client_dict_iterate_context {
 	struct dict_iterate_context ctx;
+	char *error;
 
 	pool_t pool;
-	bool failed;
 	bool finished;
 };
 
@@ -655,10 +655,8 @@ client_dict_iterate_init(struct dict *_dict, const char *const *paths,
 			str_append(query, dict_client_escape(paths[i]));
 	}
 	str_append_c(query, '\n');
-	if (client_dict_send_query(dict, str_c(query), &error) < 0) {
-		i_error("%s", error);
-		ctx->failed = TRUE;
-	}
+	if (client_dict_send_query(dict, str_c(query), &error) < 0)
+		ctx->error = i_strdup(error);
 	return &ctx->ctx;
 }
 
@@ -671,13 +669,12 @@ static bool client_dict_iterate(struct dict_iterate_context *_ctx,
 	char *line, *key, *value;
 	const char *error;
 
-	if (ctx->failed)
+	if (ctx->error != NULL)
 		return FALSE;
 
 	/* read next reply */
 	if (client_dict_read_line(dict, &line, &error) < 0) {
-		i_error("%s", error);
-		ctx->failed = TRUE;
+		ctx->error = i_strdup(error);
 		return FALSE;
 	}
 
@@ -696,7 +693,7 @@ static bool client_dict_iterate(struct dict_iterate_context *_ctx,
 		value = strchr(key, '\t');
 		break;
 	case DICT_PROTOCOL_REPLY_FAIL:
-		ctx->failed = TRUE;
+		ctx->error = i_strdup_printf("dict-server returned failure: %s", line+1);
 		return FALSE;
 	default:
 		key = NULL;
@@ -705,8 +702,7 @@ static bool client_dict_iterate(struct dict_iterate_context *_ctx,
 	}
 	if (value == NULL) {
 		/* broken protocol */
-		i_error("dict client (%s) sent broken iterate reply: %s", dict->path, line);
-		ctx->failed = TRUE;
+		ctx->error = i_strdup_printf("dict client (%s) sent broken iterate reply: %s", dict->path, line);
 		return FALSE;
 	}
 	*value++ = '\0';
@@ -716,17 +712,20 @@ static bool client_dict_iterate(struct dict_iterate_context *_ctx,
 	return TRUE;
 }
 
-static int client_dict_iterate_deinit(struct dict_iterate_context *_ctx)
+static int client_dict_iterate_deinit(struct dict_iterate_context *_ctx,
+				      const char **error_r)
 {
 	struct client_dict *dict = (struct client_dict *)_ctx->dict;
 	struct client_dict_iterate_context *ctx =
 		(struct client_dict_iterate_context *)_ctx;
-	int ret = ctx->failed ? -1 : 0;
+	int ret = ctx->error != NULL ? -1 : 0;
 
 	if (!ctx->finished)
 		dict->iter_replies_skip++;
 
+	*error_r = t_strdup(ctx->error);
 	pool_unref(&ctx->pool);
+	i_free(ctx->error);
 	i_free(ctx);
 	dict->in_iteration = FALSE;
 
diff --git a/src/lib-dict/dict-file.c b/src/lib-dict/dict-file.c
index ee735a4040..856453c625 100644
--- a/src/lib-dict/dict-file.c
+++ b/src/lib-dict/dict-file.c
@@ -46,7 +46,7 @@ struct file_dict_iterate_context {
 	struct file_dict_iterate_path *paths;
 
 	enum dict_iterate_flags flags;
-	unsigned int failed:1;
+	const char *error;
 };
 
 static struct dotlock_settings file_dict_dotlock_settings = {
@@ -232,10 +232,8 @@ file_dict_iterate_init(struct dict *_dict, const char *const *paths,
 	ctx->flags = flags;
 	ctx->iter = hash_table_iterate_init(dict->hash);
 
-	if (file_dict_refresh(dict, &error) < 0) {
-		i_error("%s", error);
-		ctx->failed = TRUE;
-	}
+	if (file_dict_refresh(dict, &error) < 0)
+		ctx->error = p_strdup(pool, error);
 	return &ctx->ctx;
 }
 
@@ -284,12 +282,14 @@ static bool file_dict_iterate(struct dict_iterate_context *_ctx,
 	return FALSE;
 }
 
-static int file_dict_iterate_deinit(struct dict_iterate_context *_ctx)
+static int file_dict_iterate_deinit(struct dict_iterate_context *_ctx,
+				    const char **error_r)
 {
 	struct file_dict_iterate_context *ctx =
 		(struct file_dict_iterate_context *)_ctx;
-	int ret = ctx->failed ? -1 : 0;
+	int ret = ctx->error != NULL ? -1 : 0;
 
+	*error_r = t_strdup(ctx->error);
 	hash_table_iterate_deinit(&ctx->iter);
 	pool_unref(&ctx->pool);
 	return ret;
diff --git a/src/lib-dict/dict-private.h b/src/lib-dict/dict-private.h
index 3d1c89bbab..16893e8ac4 100644
--- a/src/lib-dict/dict-private.h
+++ b/src/lib-dict/dict-private.h
@@ -19,7 +19,8 @@ struct dict_vfuncs {
 				enum dict_iterate_flags flags);
 	bool (*iterate)(struct dict_iterate_context *ctx,
 			const char **key_r, const char **value_r);
-	int (*iterate_deinit)(struct dict_iterate_context *ctx);
+	int (*iterate_deinit)(struct dict_iterate_context *ctx,
+			      const char **error_r);
 
 	struct dict_transaction_context *(*transaction_init)(struct dict *dict);
 	int (*transaction_commit)(struct dict_transaction_context *ctx,
diff --git a/src/lib-dict/dict-sql.c b/src/lib-dict/dict-sql.c
index 58bae58a8c..dcaea55ee1 100644
--- a/src/lib-dict/dict-sql.c
+++ b/src/lib-dict/dict-sql.c
@@ -46,7 +46,7 @@ struct sql_dict_iterate_context {
 	unsigned int key_prefix_len, pattern_prefix_len, next_map_idx;
 	unsigned int path_idx, sql_fields_start_idx;
 	bool synchronous_result;
-	bool failed;
+	const char *error;
 };
 
 struct sql_dict_inc_row {
@@ -653,10 +653,9 @@ sql_dict_iterate_init(struct dict *_dict, const char *const *paths,
 
 	ctx->key = str_new(pool, 256);
 	if (sql_dict_iterate_next_query(ctx, &error) <= 0) {
-		i_error("sql dict iterate failed for %s: %s",
-			paths[0], error);
+		ctx->error = p_strdup_printf(pool,
+			"sql dict iterate failed for %s: %s", paths[0], error);
 		ctx->result = NULL;
-		ctx->failed = TRUE;
 		return &ctx->ctx;
 	}
 	return &ctx->ctx;
@@ -672,7 +671,7 @@ static bool sql_dict_iterate(struct dict_iterate_context *_ctx,
 	int ret;
 
 	_ctx->has_more = FALSE;
-	if (ctx->failed)
+	if (ctx->error != NULL)
 		return FALSE;
 
 	for (;;) {
@@ -695,8 +694,8 @@ static bool sql_dict_iterate(struct dict_iterate_context *_ctx,
 			return FALSE;
 	}
 	if (ret < 0) {
-		ctx->failed = TRUE;
-		i_error("dict sql iterate failed: %s",
+		ctx->error = p_strdup_printf(ctx->pool,
+			"dict sql iterate failed: %s",
 			sql_result_get_error(ctx->result));
 		return FALSE;
 	}
@@ -733,12 +732,14 @@ static bool sql_dict_iterate(struct dict_iterate_context *_ctx,
 	return TRUE;
 }
 
-static int sql_dict_iterate_deinit(struct dict_iterate_context *_ctx)
+static int sql_dict_iterate_deinit(struct dict_iterate_context *_ctx,
+				   const char **error_r)
 {
 	struct sql_dict_iterate_context *ctx =
 		(struct sql_dict_iterate_context *)_ctx;
-	int ret = ctx->failed ? -1 : 0;
+	int ret = ctx->error != NULL ? -1 : 0;
 
+	*error_r = t_strdup(ctx->error);
 	if (ctx->result != NULL)
 		sql_result_unref(ctx->result);
 	pool_unref(&ctx->pool);
diff --git a/src/lib-dict/dict.c b/src/lib-dict/dict.c
index 45fd8017b0..e8c4123650 100644
--- a/src/lib-dict/dict.c
+++ b/src/lib-dict/dict.c
@@ -144,7 +144,6 @@ dict_iterate_init_multiple(struct dict *dict, const char *const *paths,
 		i_assert(dict_key_prefix_is_valid(paths[i]));
 	if (dict->v.iterate_init == NULL) {
 		/* not supported by backend */
-		i_error("%s: dict iteration not supported", dict->name);
 		return &dict_iter_unsupported;
 	}
 	return dict->v.iterate_init(dict, paths, flags);
@@ -170,13 +169,17 @@ bool dict_iterate_has_more(struct dict_iterate_context *ctx)
 	return ctx->has_more;
 }
 
-int dict_iterate_deinit(struct dict_iterate_context **_ctx)
+int dict_iterate_deinit(struct dict_iterate_context **_ctx,
+			const char **error_r)
 {
 	struct dict_iterate_context *ctx = *_ctx;
 
 	*_ctx = NULL;
-	return ctx == &dict_iter_unsupported ? -1 :
-		ctx->dict->v.iterate_deinit(ctx);
+	if (ctx == &dict_iter_unsupported) {
+		*error_r = "Dict doesn't support iteration";
+		return -1;
+	}
+	return ctx->dict->v.iterate_deinit(ctx, error_r);
 }
 
 struct dict_transaction_context *dict_transaction_begin(struct dict *dict)
diff --git a/src/lib-dict/dict.h b/src/lib-dict/dict.h
index 7b674ca068..406f8209d7 100644
--- a/src/lib-dict/dict.h
+++ b/src/lib-dict/dict.h
@@ -96,7 +96,7 @@ bool dict_iterate_has_more(struct dict_iterate_context *ctx);
 bool dict_iterate(struct dict_iterate_context *ctx,
 		  const char **key_r, const char **value_r);
 /* Returns 0 = ok, -1 = iteration failed */
-int dict_iterate_deinit(struct dict_iterate_context **ctx);
+int dict_iterate_deinit(struct dict_iterate_context **ctx, const char **error_r);
 
 /* Start a new dictionary transaction. */
 struct dict_transaction_context *dict_transaction_begin(struct dict *dict);
diff --git a/src/lib-fs/fs-dict.c b/src/lib-fs/fs-dict.c
index 4c4653b5d5..9132fcaf85 100644
--- a/src/lib-fs/fs-dict.c
+++ b/src/lib-fs/fs-dict.c
@@ -282,11 +282,12 @@ static const char *fs_dict_iter_next(struct fs_iter *_iter)
 static int fs_dict_iter_deinit(struct fs_iter *_iter)
 {
 	struct dict_fs_iter *iter = (struct dict_fs_iter *)_iter;
+	const char *error;
 	int ret;
 
-	ret = dict_iterate_deinit(&iter->dict_iter);
+	ret = dict_iterate_deinit(&iter->dict_iter, &error);
 	if (ret < 0)
-		fs_set_error(_iter->fs, "Dict iteration failed");
+		fs_set_error(_iter->fs, "Dict iteration failed: %s", error);
 	i_free(iter);
 	return ret;
 }
diff --git a/src/lib-storage/index/index-attribute.c b/src/lib-storage/index/index-attribute.c
index cbba545a29..1c07a6cb3d 100644
--- a/src/lib-storage/index/index-attribute.c
+++ b/src/lib-storage/index/index-attribute.c
@@ -302,13 +302,17 @@ int index_storage_attribute_iter_deinit(struct mailbox_attribute_iter *_iter)
 {
 	struct index_storage_attribute_iter *iter =
 		(struct index_storage_attribute_iter *)_iter;
+	const char *error;
 	int ret;
 
 	if (iter->diter == NULL) {
 		ret = iter->dict_disabled ? 0 : -1;
 	} else {
-		if ((ret = dict_iterate_deinit(&iter->diter)) < 0)
-			mail_storage_set_internal_error(_iter->box->storage);
+		if ((ret = dict_iterate_deinit(&iter->diter, &error)) < 0) {
+			mail_storage_set_critical(_iter->box->storage,
+				"dict_iterate(%s) failed: %s",
+				iter->prefix, error);
+		}
 	}
 	i_free(iter->prefix);
 	i_free(iter);
diff --git a/src/plugins/acl/acl-lookup-dict.c b/src/plugins/acl/acl-lookup-dict.c
index add5c0ffb2..34e415cd47 100644
--- a/src/plugins/acl/acl-lookup-dict.c
+++ b/src/plugins/acl/acl-lookup-dict.c
@@ -155,6 +155,7 @@ acl_lookup_dict_rebuild_update(struct acl_lookup_dict *dict,
 	struct dict_iterate_context *iter;
 	struct dict_transaction_context *dt;
 	const char *prefix, *key, *value, *const *old_ids, *const *new_ids, *p;
+	const char *error;
 	ARRAY_TYPE(const_string) old_ids_arr;
 	unsigned int newi, oldi, old_count, new_count;
 	string_t *path;
@@ -179,8 +180,8 @@ acl_lookup_dict_rebuild_update(struct acl_lookup_dict *dict,
 			array_append(&old_ids_arr, &key, 1);
 		}
 	}
-	if (dict_iterate_deinit(&iter) < 0) {
-		i_error("acl: dict iteration failed, can't update dict");
+	if (dict_iterate_deinit(&iter, &error) < 0) {
+		i_error("acl: dict iteration failed: %s - can't update dict", error);
 		return -1;
 	}
 
@@ -264,7 +265,7 @@ int acl_lookup_dict_rebuild(struct acl_lookup_dict *dict)
 static void acl_lookup_dict_iterate_read(struct acl_lookup_dict_iter *iter)
 {
 	struct dict_iterate_context *dict_iter;
-	const char *const *idp, *prefix, *key, *value;
+	const char *const *idp, *prefix, *key, *value, *error;
 	unsigned int prefix_len;
 
 	idp = array_idx(&iter->iter_ids, iter->iter_idx);
@@ -288,8 +289,10 @@ static void acl_lookup_dict_iterate_read(struct acl_lookup_dict_iter *iter)
 		key = p_strdup(iter->iter_value_pool, key + prefix_len);
 		array_append(&iter->iter_values, &key, 1);
 	}
-	if (dict_iterate_deinit(&dict_iter) < 0)
+	if (dict_iterate_deinit(&dict_iter, &error) < 0) {
+		i_error("%s", error);
 		iter->failed = TRUE;
+	}
 }
 
 struct acl_lookup_dict_iter *
diff --git a/src/plugins/expire/doveadm-expire.c b/src/plugins/expire/doveadm-expire.c
index ea2e12d31a..51e0dd84b3 100644
--- a/src/plugins/expire/doveadm-expire.c
+++ b/src/plugins/expire/doveadm-expire.c
@@ -126,7 +126,7 @@ doveadm_expire_mail_cmd_get_next_user(struct doveadm_mail_cmd_context *ctx,
 {
 	struct doveadm_expire_mail_cmd_context *ectx =
 		DOVEADM_EXPIRE_MAIL_CMD_CONTEXT(ctx);
-	const char *key, *value;
+	const char *key, *value, *error;
 	unsigned long oldest_savedate;
 	int ret;
 
@@ -159,8 +159,8 @@ doveadm_expire_mail_cmd_get_next_user(struct doveadm_mail_cmd_context *ctx,
 	}
 
 	/* finished */
-	if (dict_iterate_deinit(&ectx->iter) < 0) {
-		i_error("Dictionary iteration failed");
+	if (dict_iterate_deinit(&ectx->iter, &error) < 0) {
+		i_error("Dictionary iteration failed: %s", error);
 		return -1;
 	}
 	return 0;
@@ -352,10 +352,11 @@ static void doveadm_expire_mail_cmd_deinit(struct doveadm_mail_cmd_context *ctx)
 {
 	struct doveadm_expire_mail_cmd_context *ectx =
 		DOVEADM_EXPIRE_MAIL_CMD_CONTEXT(ctx);
+	const char *error;
 
 	if (ectx->iter != NULL) {
-		if (dict_iterate_deinit(&ectx->iter) < 0)
-			i_error("expire: Dictionary iteration failed");
+		if (dict_iterate_deinit(&ectx->iter, &error) < 0)
+			i_error("expire: Dictionary iteration failed: %s", error);
 	}
 	if (dict_transaction_commit(&ectx->trans) < 0)
 		i_error("expire: Dictionary commit failed");