@Roland I had the same problem, a reboot of the phone helped.
Additionally, I set the option to distrust any certificate and upon a manual sync, I was asked about the certificated and then again for the OpenTasks permission.
This is the https://github.com/MirakelX/davdroid fork. Not sure how/whether applicable it is for DAVDroid main, but did not manage to find out how to file issue against fork. So this may be one to assign elsewhere.
In any case, I consistently see the sync aborting after 404ing on the same task. When I enter the URL that fails into the browser, it downloads it without problem:
The error is when PUTting the resource, not when GETting it as your browser does. However, please report bugs for DAVdroid-Mirakel to http://azapps.de/kontakt-und-datenschutz/
Issue tracker is here.
I will take a look at this.
OK thanks, please follow up there.