Thank you for reporting.
Jottacloud never published an official API, but it has been reverse engineered and used in different projects, since maybe 2014. They are obviously not opposed to such use even if not officially endorsing it, e.g. based on the fact that they re-enabled legacy auth after Duplicati user’s feedback, and in the past they even encouraged it in the old forum.
Its been working in Duplicati since 2017 and in rclone since 2018, in rclone with the new authentication since end of 2019. I think there is a chance we can get a working Jottacloud backend in Dupliati using the current OpenID/OAuth based authentication (#4699 and #7). Even though we have no guarantees, I would expect that to work for years to come. Its not like Jottacloud does changes very often, see their blog for instance.