Sharing broken after migration/Update

Hello everyone,

i have a strange Problem: last NIght i did the following Things:

  1. migrate an 2 Year old owncloud 9.1.4 to nextcloud 10 on ubuntu 14.04 server 64bit
  2. after that semmd to run well, i upgraded the ubuntu server to 16.04 since nc 11 needed a higher php Version than 14.04 was providing
  3. upgraded nc to 11

Then i sent a few Users the request to check if the find Issues.
One wrote today: the Shared Files he “received” in the Past are all “broken”.
e.g. he had the File document.odt which another user shared with him in the Past.
Now it read document.odt(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)(2)
with multiple (2)(2)… at the End of the Filename.
The Document was not usable anymore.
He said all the Files shared with him are broken in such a way.
I have two Files other Users shared with me: those are not broken.

One of the files he has was shared by me.
I “unshared” it an shared it again which fixed the issue for this one File…

My Question is: has someone seen that Problem before?
If nothing helps: can i, as the administrator, unshare alle Files within the cloud?