Mail Loop Email Deliverability Broken Since mid 2.3.0

CMK version: 2.4.0p2
OS version: Docker

Error message:

Unhandled exception: RuntimeError('Either Username/Passwort or ClientID/ClientSecret/TenantID have to be set')

This is the file giving the error: /opt/omd/versions/2.4.0p2.cce/lib/python3/cmk/plugins/emailchecks/lib/ac_args.py

The service worked properly before, there were no changes to the mail server / test user.