I had the problem that backups on a local harddrive worked fine and the same source folder as a backup to a Synology NAS over webdav failed with the error message “Missing files on the remote storage, please run repair”. The repair then failed also. BUT: All files were transferred to the remote NAS a were stored without errors.
The problem is the path you type into the field for the backup-location for a (in my case) linux-based remote folder on the Synology NAS.
You have to write a slash / instead of a backslash \ for a subfolder.
Maybe everyone already knows that, maybe it’s a bug. I didn’t know it and it took me a lot of nerves to find out.