RemoteServiceException: Bad notification for statForeground - no notifications in app

I have encountered a strange error. For some time this crash has been popping up in my Fabric

Fatal Exception: android.app.RemoteServiceException: Bad notification for startForeground: java.lang.RuntimeException: invalid channel for service notification: Notification(channel=null pri=-2 contentView=null vibrate=null sound=null defaults=0x0 flags=0x40 color=0x00000000 vis=PRIVATE)

I understand that it's an error saying my notification has no channel defined, which is required since Android 8 or 8.1 (all the crashes are on 8+), but the problem is that I don't use any notifications in whole project. Could this be a problem from some external library I use ? And if so, why on earth are some external libraries doing something with notifications ...

Could this be a problem from some external library I use ?

Yes.

why on earth are some external libraries doing something with notifications ...

Based on your error, they are starting a foreground service, and failing to set up a proper Notification for it (e.g., did not associate it with a notification channel).

You should be able to look at your merged manifest (e.g., Manifest Merger sub-tab in Android Studio when editing a manifest) to see what libraries are contributing services, to try to narrow down the source of the problem. Then, either update to a newer version of that library or try to get rid of the library.