Just got another response from Jottacloud support:
“Hi!
We are looking into the issue Duplicati users are reporting regarding authentication, to see if we can make a temporary fix as a one-off for this specific issue. No change in our policy regarding support of third-party apps/services.”
So maybe it is not all lost after all, except that in time of course Duplicati would have to be able to work without this “temporary fix”…