It looks like you’re using the most stable version of Duplicati available at the moment, and I know other users have been able to run it on Open Media Vault so I doubt the issue is there…
And everything looks OK to me in the config and the error message just tells us that JottaCloud considers Duplicati unauthorized. Unfortunately, I don’t think we can get any more detail from JottaCloud about WHY they consider it unauthorized.
Is it possible it has stale or missing SSL certificates (which are destination independent) as seemed to have been the issue here?