Fetchmail is not working, fetchmailrc untouched

Hi, I upgraded UCS to version 5.02, and all was fine then. but after some package updates I was not able to fetch E-Mails for new users via fetchmail. I notice that the fetchmailrc is not updated no matter what I fill in the forms under User.
When I manually insert the information in the fetchmailrc everything works as expected. I learn it has something to do with the listener, and in the listener.log I find

Traceback (most recent call last):
  File "/usr/lib/univention-directory-listener/system/fetchmailrc.py", line 163, in handler
    old = p.load()
TypeError: a bytes-like object is required, not 'str'
02.09.22 13:11:17.057  LISTENER    ( WARN    ) : handler: fetchmailrc (failed)

Does anybody has any ideas?
Thanks in advance
Christoph