Thank you kindly for the responses and the very informative details. I think you are correct that something is faulty with the WebDAV instance.
I wish I could disable SSL for testing purposes, but the instance is hosted outside my control.
Investigating as per your advice, I now found that even “Test connection” throws the “Root element is missing”.
Things may go awry before “debug-propfind-file” has a chance to trigger. At least I didn’t see any output to this file while testing.
Testing the list command with Duplicati.CommandLine.BackendTool.exe triggers the “Root element is missing” message before listing any files.
Finally, I mapped the WebDAV path directly in Windows - which absolutely should work - and this too timed out before listing any files…
I think I owe Duplicati an apology for this one.
To sum up:
I have several Duplicati backups that connects to the same Nextcloud instance via the same settings using WebDAV. They all work, except the one above which suddenly started throwing the “Root element is missing” error.
The only obvious difference between them, is that the faulty backup is close to 4TB of data and the others are <10GB each.
Perhaps the Nextcloud instance is choking on listing the 80.000 or so files (volume size of 50MByte).
I think my next attempt will be to recreate the backup using a much larger volume size.
Is there any information I can provide that would be useful for Duplicati development? Realising that this really does look like an external issue.
Once again, thank you for all your help.