Hint: This article is not suitable anymore for UCS5 !
Problem
Your portal site is empty caused by an error or mistake.
Environment
UCS: 4.4-8
Solution
Please try to remove or delete:
mv /var/cache/univention-portal/portal.json /root/portal.jso_bak
restart your Portalserver:
kill -HUP $(ps aux | grep '[u]nivention-portal-server' | awk '{print $2 }')
you might check if the portal.json was recreated:
ls -latr /var/cache/univention-portal/
or go straight ahead to your site.
If there is a groups,json in your /var/cache/univention-portal/ and your portal ist still not working, keep this bug in mind.
Do the following steps with the attached python-File :
rebuild-portal-groups.py (1.2 KB)
root@kochmaa:~# systemctl stop univention-directory-listener
root@kochmaa:~#./rebuild-portal-groups > groups.json
—> Please Check the file groups.json 
root@kochmaa:~# chgrp nogroup groups.json
root@kochmaa:~# mv groups.json /var/cache/univention-portal/groups.json
root@kochmaa:~# systemctl start univention-directory-listener
Delete:
/root/portal.jso_bak
If this was not sufficient it could be useful to retrigger the listener modul
univention-directory-listener-ctrl resync portal_groups
Check also:
UCS Portal broken after last errata updates
Backing up UCS 4.4, everything working except portal edition
Portal entries disappear when restricted to groups
Einträge im Portal nicht sichtbar
Kudos for the Solution by @Moritz_Bunkus