diff --git a/src/filter_core/filter_session.c b/src/filter_core/filter_session.c
index ccb04e01d8..34ffbdf3cc 100644
--- a/src/filter_core/filter_session.c
+++ b/src/filter_core/filter_session.c
@@ -1583,8 +1583,13 @@ void gf_fs_print_debug_info(GF_FilterSession *fsess, GF_SessionDebugFlag dbg_fla
 //this defines the sleep time for this case
 #define MONOTH_MIN_SLEEP	5
 
-#if defined(GPAC_CONFIG_EMSCRIPTEN) && !defined(GPAC_DISABLE_THREADS)
+#if defined(GPAC_CONFIG_EMSCRIPTEN)
+// && !defined(GPAC_DISABLE_THREADS)
 #include <emscripten/threading.h>
+#endif
+
+#if defined(GPAC_CONFIG_EMSCRIPTEN) && !defined(GPAC_DISABLE_THREADS)
+//#include <emscripten/threading.h>
 GF_Err gf_th_async_call(GF_Thread *t, u32 (*Run)(void *param), void *param);
 static u32 gf_fs_thread_proc(GF_SessionThread *sess_thread);
 
@@ -1631,7 +1636,8 @@ static u32 gf_fs_thread_proc(GF_SessionThread *sess_thread)
 	Bool skip_next_sema_wait = GF_FALSE;
 	GF_Filter *current_filter = NULL;
 
-#if defined(GPAC_CONFIG_EMSCRIPTEN) && !defined(GPAC_DISABLE_THREADS)
+#if defined(GPAC_CONFIG_EMSCRIPTEN)
+// && !defined(GPAC_DISABLE_THREADS)
 	Bool flush_main_blocking = (!thid && !fsess->is_worker) ? GF_TRUE : GF_FALSE;
 	if (flush_main_blocking) do_regulate = GF_FALSE;
 #endif
@@ -1696,9 +1702,12 @@ static u32 gf_fs_thread_proc(GF_SessionThread *sess_thread)
 		}
 #endif
 
-#if defined(GPAC_CONFIG_EMSCRIPTEN) && !defined(GPAC_DISABLE_THREADS)
-		if (flush_main_blocking)
-			emscripten_main_thread_process_queued_calls();
+#if defined(GPAC_CONFIG_EMSCRIPTEN)
+//&& !defined(GPAC_DISABLE_THREADS)
+		if (flush_main_blocking) {
+//			emscripten_main_thread_process_queued_calls();
+			emscripten_current_thread_process_queued_calls();
+		}
 #endif
 
 		safe_int_dec(&fsess->active_threads);
@@ -4051,6 +4060,14 @@ static GF_DownloadManager *gf_fs_get_download_manager(GF_FilterSession *fs)
 {
 #ifdef GPAC_USE_DOWNLOADER
 	if (!fs->download_manager) {
+
+#ifdef GPAC_CONFIG_EMSCRIPTEN
+		if (!fs->non_blocking) {
+			GF_LOG(GF_LOG_ERROR, GF_LOG_HTTP, ("Fetch cannot be used in blocking mode as it requires returning to main/worker JS event loop\n"));
+			return NULL;
+		}
+#endif
+
 		fs->download_manager = gf_dm_new(fs);
 
 #ifndef GPAC_DISABLE_NETWORK
diff --git a/src/filters/decrypt_cenc_isma.c b/src/filters/decrypt_cenc_isma.c
index 7e0a848b8c..bc53fbec47 100644
--- a/src/filters/decrypt_cenc_isma.c
+++ b/src/filters/decrypt_cenc_isma.c
@@ -2,7 +2,7 @@
  *			GPAC - Multimedia Framework C SDK
  *
  *			Authors: Jean Le Feuvre
- *			Copyright (c) Telecom ParisTech 2018-2023
+ *			Copyright (c) Telecom ParisTech 2018-2024
  *					All rights reserved
  *
  *  This file is part of GPAC / CENC and ISMA decrypt filter
@@ -815,6 +815,7 @@ static GF_Err cenc_dec_set_clearkey(GF_CENCDecCtx *ctx, GF_CENCDecStream *cstr,
 
 #ifdef GPAC_USE_DOWNLOADER
 	GF_DownloadManager *dm = gf_filter_get_download_manager(ctx->filter);
+	if (!dm) return GF_SERVICE_ERROR;
 	cstr->sess = gf_dm_sess_new(dm, ck_url, 0, ck_http_io, cstr, &e);
 	if (e) return e;
 	ctx->pending_keys++;
@@ -952,6 +953,7 @@ static GF_Err cenc_dec_set_hls_key(GF_CENCDecCtx *ctx, GF_CENCDecStream *cstr, c
 		GF_Err e = GF_SERVICE_ERROR;
 #ifdef GPAC_USE_DOWNLOADER
 		GF_DownloadManager *dm = gf_filter_get_download_manager(ctx->filter);
+		if (!dm) return GF_SERVICE_ERROR;
 		GF_DownloadSession *sess = gf_dm_sess_new(dm, key_url, GF_NETIO_SESSION_NOT_CACHED, hls_kms_io, cstr, &e);
 		if (e) {
 			GF_LOG(GF_LOG_ERROR, GF_LOG_DASH, ("[CENC/HLS] Failed to setup download session for key %s: %s\n", key_url, gf_error_to_string(e)))
diff --git a/src/filters/in_http.c b/src/filters/in_http.c
index 7447162239..ad718500bd 100644
--- a/src/filters/in_http.c
+++ b/src/filters/in_http.c
@@ -2,7 +2,7 @@
  *			GPAC - Multimedia Framework C SDK
  *
  *			Authors: Jean Le Feuvre
- *			Copyright (c) Telecom ParisTech 2017-2023
+ *			Copyright (c) Telecom ParisTech 2017-2024
  *					All rights reserved
  *
  *  This file is part of GPAC / HTTP input filter using GPAC http stack
@@ -106,9 +106,8 @@ static GF_Err httpin_initialize(GF_Filter *filter)
 
 	if (!ctx || !ctx->src) return GF_BAD_PARAM;
 	ctx->dm = gf_filter_get_download_manager(filter);
-#ifndef GPAC_CONFIG_EMSCRIPTEN
 	if (!ctx->dm) return GF_SERVICE_ERROR;
-#endif
+
 	ctx->block = gf_malloc(ctx->block_size +1);
 
 	flags = GF_NETIO_SESSION_NOT_THREADED | GF_NETIO_SESSION_PERSISTENT;
diff --git a/src/filters/io_fcryp.c b/src/filters/io_fcryp.c
index d9ffc50979..432585c48e 100644
--- a/src/filters/io_fcryp.c
+++ b/src/filters/io_fcryp.c
@@ -2,7 +2,7 @@
  *			GPAC - Multimedia Framework C SDK
  *
  *			Authors: Jean Le Feuvre
- *			Copyright (c) Telecom ParisTech 2021-2023
+ *			Copyright (c) Telecom ParisTech 2021-2024
  *					All rights reserved
  *
  *  This file is part of GPAC / file crypt/decrypt for full segment encryption filter
@@ -244,6 +244,11 @@ static GF_Err cryptfin_process(GF_Filter *filter)
 			//use a threaded session
 			if (!ctx->key_sess) {
 				GF_DownloadManager *dm = gf_filter_get_download_manager(filter);
+				if (!dm) {
+					ctx->in_error = GF_NOT_SUPPORTED;
+					ctx->reload_key_state = KEY_STATE_NONE;
+					return ctx->in_error;
+				}
 				ctx->key_sess = gf_dm_sess_new(dm, ctx->key_url, GF_NETIO_SESSION_NOT_CACHED, cryptfin_net_io, ctx, &e);
 			} else {
 				e = gf_dm_sess_setup_from_url(ctx->key_sess, ctx->key_url, GF_TRUE);