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You need to set the log level to "All information and debug", then run the backup again. This will give a much more detailed log file. Send me that log file so I can help you further.

Yes, the "All Information and Debug" is really what it says in the tin. Very, very verbose, but most critically it has the debugging information which allows me to trace the problem back to its source.

While your connection information is correct, the Directory you have provided does not exist on your WebDAV server.

It is very likely that you need to enter a specific Directory in a specific format, or the Endpoint may require a suffix.

For example, when using NextCloud the Endpoint URL is something like https://example.com/nextcloud/remote.php/dav/files/myusername and the Directory for the root is simply /. If you forget the /dav/files/myusername after remote.php it won't work – I assume, without having explicitly tried, even if you added /dav/files/myusername as the Directory since you cannot connect to remote.php as a WebDAV endpoint.

When using a Synology NAS, on the other hand, the Endpoint URL would be something like http://192.0.2.123:5006 with a Directory that matches the name of the share you are uploading to, e.g. /foobar. Trying to use / as the Directory will fail as the root folder in its WebDAV implementation is not a real filesystem directory (it's virtual), thus non-writeable.

I know it's all a bit confusing. WebDAV is second only to FTP when it comes to the amount of confusion it causes. The worst part is that most instructions tell you "use this endpoint and directory for software X" taking only into account the quirky way that particular software parses a WebDAV endpoint URL, and not how the WebDAV specification is supposed to work.