Correct, I posted the same problem already half a year ago.
Unfortunately I did not find a solution then and had to leave it due to lack of time.
With the latest update it popped up again. I spent several days around new year to fix it, I think I tried every suggested solution on the forum - without success.
I checked / tried these:
Troubleshooting: Listener-/Notifier
Howto Check for Listener/ Notifier Service Status
What to do if a failed.ldif is found
Problem: Unable to (re-)join: 03univention-directory-listener.inst failed
Replication issues UDN
Problems with UDN Replication - Notifier ID and local ID do not match
MDB Database Full and Stops Replication
How to reset Listener / Notifier replication
Still I did not manage to fix. This is why I ask for help again.