For example an error like this:
PUT locks/exclusive_desktop_37460a5a86c54c3184070f7c64c6a1d6.json: “Joplin/locks/exclusive_desktop_37460a5a86c54c3184070f7c64c6a1d6.json” is locked (Exception OCA\DAV\Connector\Sabre\Exception\FileLocked) (423): <?xml version="1.0" encoding="utf-8"?> <d:error xmlns:d=“DAV:” xmlns:s=“http://sabredav.org/ns”> <s:exception>OCA\DAV\Connector\Sabre\Exception\FileLocked</s:exception> <s:message>“Joplin/locks/exclusive_desktop_37460a5a86c54c3184070f7c64c6a1d6.json” is locked</s:message> </d:error>
This is a Nextcloud issue (Nextcloud is broken by default) and you need to head to their forum for support.
Most likely you need to switch to Redis for file caching and locking but again you'll get more details on their forum.