Sample configuration code below
https://doc.dovecot.org/2.4.2/core/config/dict.html#sql-mapping
is wrong. Setting value_field doesn't exit. Most likely should be value (resp. dict_map_value).
The next code block some lines further down contains key_field which is not mentioned in the rest of the documentation (nor dict_map_key_field). Here it is the other way round: key_field (resp. dict_map_key_field) is correct, whereas field (resp. dict_map_field) is not accepted.