Historically, that’s not proof of a correct URL, for example see:
Unable to backup after upgrade to 2.1.0.4_stable
where upload works but list doesn’t. It looks like code fix caused compatibility problems.
WebDAV list on Duplicati 2.1.0.5 gives 405, but works with 2.1.0.4 #6015
Based on the clues from #6029 I think the problem is that 2.1.0.4 had a bug that would remove a slash.
Warn on leading slashes for some providers #187 tries to stop people from inviting trouble, however it’s in the new UI which is out for Canary test. Bottom line is URLs can be trouble.
Suggestion remains to check URL and test variations if possible. Info on WebDAV server is potentially helpful, as we’re not hearing about this type of problem from other users AFAIK.
You can also do extended testing with BackendTool to try to find out what works or doesn’t.
If the WebDAV server is safe to use with unencrypted HTTP, looking at its traffic might help.
Possibly the developer who worked on previous URL or WebDAV problems will have ideas.