Although the error looks similar, it is often best to create a new thread instead of posting in an old one.
sry for that 
Regarding the error message: When was the system installed? We had a small time window where a bug caused an incomplete initialization of a background database for the school import, see Bug 53599
I installed this system 2 weeks ago on July 6th.
Please test if re-running the joinscript fixes the issue:
univention-run-join-scripts --run-scripts 40ucs-school-import-http-api.inst --force
Unfortunately the behaviour stays the same. Tried it with restarting the relevant services as described here Problem: Nach dem Installieren kann das Import Modul nicht aufgerufen werden
br,
Manuel