Thank you!!! Deleting portal.json literally fixed everything!
This portal.json got there because that’s in one of the steps of “Single Server Backup and Restore”:
After that, you must rebuild the UMC portal page. Otherwise you have double portal page entries.
cp /usr/share/univention-portal/portal-unjoined.json /usr/share/univention-portal/portal.json
univention-directory-listener-ctrl resync portal
univention-directory-listener-ctrl resync portal_entry
univention-directory-listener-ctrl resync portal_category
This renames the portal-unjoined.json (the faulty one) to portal.json, causing all the trouble.
Anyway, I’m really thankful for your help 
can’t believe it was such a simple fix to such a “complex” problem.