AD Connector Service fails after Update to 4.4

I had an UCS Server previously running version 4.3 as part of a Microsoft Active Directory Domain.

After updating to 4.4 the connector-service stopped and can’t be started again. What could be causing this? Are the any particular logfiles I should check?

I found the following error in connector-status.log

try to sync 45 changes from UCS
done: Wed Apr 15 09:16:15 2020
 --- connect failed, failure was: ---
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/univention/connector/ad/", line 302, in main
  File "/usr/lib/python2.7/dist-packages/univention/connector/ad/", line 204, in connect
  File "/usr/lib/python2.7/dist-packages/univention/connector/", line 877, in initialize_ucs
  File "/usr/lib/python2.7/dist-packages/univention/connector/", line 964, in poll_ucs
  File "/usr/lib/python2.7/dist-packages/univention/connector/", line 939, in _generate_dn_list_from
    dn, new, old, old_dn = cPickle.load(f)

please take a look on this article:
AD Connector Service fails