Mac-Client: Error processing item: Error Domain=NSCocoaErrorDomain

Hello, just installed the Mac-Client and connected to my nextcloud installation. However, when I try to share or lock a file, I’m getting this error:

Error processing item: Error Domain=NSCocoaErrorDomain Code=257 “The file couldn’t be opened because you don’t have permission to view it.”

Didn’t find anything in google or here in the forums .
Thanks for any hints!

Which version of Nextcloud are you using?
Can you find anything in the log files?

Are these normal files in your storage, or have they already been shared to you by someone else? No external storage?