Mqtt broker doesn't accept some connections

I recently rebuilt my HA installation from scratch due some issues with the DB and I decided to migrate from mosquitto installed on the main OS to the MQTT Broker component. To do this I needed to change the MQTT user for all the clients because the former installation used the user “homeassistant” now not allowed, so I created a new user in HA and I set up every device to connect to HA with that user and password.
All the smart plugs/sensors and switcher are connecting flawless since the first start, but the two mqtt apps an my two PCs are getting the “New connection from on port 1883.
error: received null username or password for unpwd check” response.
Of course user and password are set and the same apps were working on the previous mosquitto installation.
I tried to set the component with user/pwd set (everything connects but the two apps), without user/pwd set (nothing connects), with the custm folder active and inactive (no difference).
I searched on Google, but I just found some old posts here and on Reddit with the advice to do the same thing or downgrade the component (I don’t know exactly how, since there’s no version choice on the setup page).
Any idea?
(since the sensors/switches are working I can surely live without knowing the cpu load for the PCs)