When I tried to upload a few photos with the Nextcloud app, I got “WebDAV error 423 file is locked” (afa I remember).
The root cause was that the creation dates of all those files in the photo library were equal (because I had created them using batch mode in the CamScanner iOS app).
The Nextcloud app tried to convert the creation date into a file name and ran into problems because the file names of multiple files to be uploaded were identical.