Try to check with ps if there is a ‘zsync’ process haning. (ps aux | grep zsync)
I’ve seen this on several setups where after updating packages and attempting to update the apps index zsync processes get stuck and never end. The updater does that for every minor version of UCS, that is 4.1 until 4.4 as of writing. One example being:
Honestly I haven’t found a better way than to ‘pkill -9 zsync’ for every stuck process. I’ve then deleted the target file the zsync command attempted to download (i.e. /var/cache/univention-appcenter/appcenter.software-univention.de/4.1/.all.tar) and then re-executed the download without “-q” to see if it succeeds later on.
Once you kill the individual zsync process the appcenter update proceeds and may get stuck on the next zsync.
You can also check /var/log/univention/appcenter.log