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Sorry to hear you’re facing problems 
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Nextcloud version (eg, 18.0.2): 20.0.4.0
Operating system and version (eg, Ubuntu 20.04): Debian
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.38
PHP version (eg, 7.1):7.3
The issue you are facing:
Is this the first time you’ve seen this error? (Y/N): Y
I am using nextcloudpi(on x86-x64). All was fine, till I update from NC 19 to NC 20.
After that I noticed that I cant delete some files. After an investigation I found that I can’t delete files that was created before update.
There is no error in logs. I can create in same folders new files and delete it.
In web interface there is no such button ‘delete’ for that old files.
When I tried to delete them from desktop client syncing, it delete it first(locally on client), then when syncing - just upload it from server back to me and says:
“Not allowed to remove, restoring”
I checked OS permissions: all is fine, ‘www-data’ is owner and permissions for new files that I can delete and for old files are the same.
Was trying to use ‘lslocks’ but there is no third party soft on server that can lock files I want to delete.
Files are not shared.
P.S: I can replace it. Just cant delete.
Can anyone help - where to dig next to find the reason?