[ENHANCEMENT]: Updated error messages for deactivated user

[ObsidianOracle]

Description

If you try to log in with a deactivated user a wrong error message appears: "Invalid Credentials". In this case it should rather say "Cannot log in. Please contact your administrator for further information".

[gluafamichl]

Backend always sends 401, we see no way to decide why the login failed in frontend, seems to be a backend issue.