advanced permissions - core exception when viewing file set

Permalink
site has 3 file sets.
non admin user logs in, can see into the first 2 file sets from dashboard/files/file sets.
on 3rd file set the user gets a core exception, see screenshot.
I have deleted that file set and created it anew, same error occurs.

Solved: there were 2 files in that 3rd file set where C5 did not show permissions when I wanted to change them back and forth. I removed those 2 files, and now user can see into 3rd file set without core exception.

1 Attachment