Now that issue T127456 was fixed, i. e. we do no longer get fatals after invoking the extension an new issue emerged, became visible:
On "Special:Preference" in the "User profile" section (mw-prefsection-personal) no group assignment is shown. This corresponds with the observation that only actions which are connected to mere existence of an account work. Actions connected to an additionally assigned user group like sysop actions etc. can no longer be performed. I must note that on "Special:ListUsers" I can still see that there is still a user group assignment existing for the user account, however not functional.