Maybe the known issue with recursion of shares discussed here:
https://github.com/owncloud/core/issues/25557
You could try 9.1.1 RC3 which should contain the fix:
https://owncloud.org/install/#testing-development
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So we have the bug with duplicated shares due to the same item being
shared by multiple group memberships.
We were gritting our teeth trying to get by until the 20th.
Last night, something bad happened and now we start getting these about
half an hour after restarting httpd:
[Fri Sep 16 14:45:29