Following your link I found https://github.com/owncloud/calendar/issues/1002 and the second comment has a solution that fixes the problem for me. I did a vimdiff on his file and without understanding much I can tell you that the changes look reasonable. I have only been testing for 5 minutes but so far it works.
Explain Tasks app update
The Tasks (referred to as OpenTasks, but called Tasks according to f-droid) app which is recommended to manage tasks doesn’t provide a way to add task lists which have been created after the server has been added in DAVDroid. It’d be nice to mention this limitation in DAVDroid.
experienced with 0.9.0.4 on Android 4.0.4