Hi @tim.adams,
With only digest auth enabled, are you still able to add files via the Windows built-in WebDAV functionality? As a baseline test, I sometimes use Cyberduck as a way to validate “standard” WebDAV functionality. Would you be able to see if Cyberduck gives the same error?
Additionally, if you can PM me some credentials, I can run some tests from my end to see if there is anything I can discover about your setup.