Thanks @Xavron for the links to the posts. Actually following ts678’s instructions here provides a working task. However I would prefer to reference the job rather by its ID so that the jobs triggered by Task Scheduler and the timer in the GUI are sure to stay in sync if I change the job setup in the GUI.
It can’t be difficult to set up the task with duplicati_client since I’m far from being experienced in such things and I managed to do it about 2 years ago 
In between I found out how to login with duplicati_client and get a list of backup jobs, so I can identify their IDs. So theoretically it would take two commands 1) login 2) trigger job which I can put in a batch file. Then I need to know how to fire up the batch file (failed so far) and so on…
So in short: it did work somehow with just the “duplicati_client run ” command in task scheduler. I just don’t remember how I solved the problem with login. I also remember that a black cmd window briefly popped up when the task started and I got a glimpse of a message like “Token expired” and “logging in” - so something must have happened which goes beyond the setup in taskscheduler. Just can’t get my head around it… 