Getting the same issue as @der_Don when completing tasks. Ghost notifications are back.
@rfc2822 have you tried installing the previous version of DavDroid and OpenTasks, creating calendars and tasks, then updating DavDroid and OpenTasks to newest versions?
EDIT: making sure to install latest DavDroid update and sync before updating OpenTasks.
My install history from F-Droid says that DavDroid was updated, and then a couple days later (I believe because of other apps I installed in between, no timestamps), OpenTasks was updated. I run the privileged extension, so.
This could ultimately be an issue with OpenTasks and not DavDroid.