Yes, we have tracked this one down to our use of setting and un-setting the immutable flag on directories to avoid the scenario where a failure to mount a subvol doesn’t end up with it’s mount point being populated via a potential share export. We have more recently fixed, in the stable channel updates, a couple of instances where the code failed to un-set these flags to the detriment of normal functions but there may be some remaining.
See the following thread where @Haioken and @Rene_Castberg reported an earlier more problematic instance of this:
which has now been sorted, and a later report by @satello of a similar instance:
which has also no be fixed in the stable channel via the pull request indicated in that thread.
Hope that helps to explain this ‘operation not permitted’ thing.