After a long time I updated our owncloud to version 10.13.4 (stable). Since this update I can no longer access SOME (not all!!) files. I get the following error via browser

The website is unavailable
The website at may be temporarily unavailable or may have been permanently moved to a new web address.

and the files cannot be accessed via sync. Can someone tell me how to fix the problem?
Thanks and greetings from Hamburg