Duplicati reported what it saw as invalid XML. Question is – what did it see? Wikipedia on Root element.
Is this remote, with SSL/TLS for security? Seeing received data would be far simpler without encryption.
EDIT:
–debug-propfind-file possibly could help, but I haven’t used it, and I also don’t have any WebDAV to try…
If testing it, you might want to start with a small backup, and find out it seems to have the file listing data.