Me88, are you still having problems with this? I may be able to help.
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