The maintenance service try to reach ldap (which works for you) and than checks if the repository is available (which seems to fail).
Unfortunately, the interesting part is cut off. Please scroll a bit to the right, and check at the line “Checking network for Univention maintenance…”
At my system it looks like
-- Reboot --
Mär 27 10:15:51 master163 systemd[1]: Starting LSB: Univention Updater...
Mär 27 10:15:52 master163 univention-maintenance[1629]: Checking network for Univention maintenance...ldap[master163.ucs.example]...repository[updates.software-univention.de]...done.
Mär 27 10:15:52 master163 systemd[1]: Started LSB: Univention Updater.
Did you block some protocols? The repository must be resolved and the check for availability is done by ICMP. You could try to reach it by
host updates.software-univention.de
ping -c2 updates.software-univention.de
Are you using a proxy service like squid for http requests? As you said there are package updates, but you can’t see them. You could try to reach it by
wget https://updates.software-univention.de/4.3/maintained/4.3-0/all/Packages.gz -O Packages.gz && md5sum Packages.gz
At my system it looks like
f97c10b409b163bd75001000a7b7d325 Packages.gz