Error 1643744696

There is permanently an error showing: component returned failure code: 0x80520015 (NS_ERROR_FILE_ACCESS_DENIED) [ns.IFile.lastModifiesTime]

But I can use Zotero more or less... When I try to add a new reference, or add a new document to Zotero's Db (usually pdf), it says that there is an error and firefox should be restarted. If I restart forefox, it is fine again (and just the above error shows)
This happens often and randomly.
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