Hi Stefano
I’ve had similiar errors in the past at my clients and at home.
What almost always worked was a bit unconventional:
Make sure you have your system backed up!
- Delete the Account Provider
- Restore an earlier config-backup
That will install all needed modules and the configuration. This solved my problem, users, groups and Account Provider was all back there!
A reboot would be in order!
My 2 cents
Andy