It is possible that the UMC gets stuck in a situation where it show no further progress after starting a software update - most likely it will only print:
... loading log file ...
(or in german
... Protokolldatei wird geladen ...)
The update itself starts successfully. You can safely monitor the running process yourself at cmd:
tail -f /var/log/univention/updater.log
If its finished, a reload of the UMC site will be sufficient to work on.