Deleted or completed tasks reappear

Hello dear community,

I have some tasks in my to-do list that I can't seem to get rid of. Whether I delete them or mark them as completed, they reappear after a few seconds/minutes.

The tasks are basically from the planner, but they have already disappeared there, i.e. deleted.

I have already logged out of To-Do several times, reinstalled To-Do on my PC, and even tried to get rid of the tasks through the web app, but unfortunately, nothing has worked.

It's worth mentioning that I also have sync problems on my phone. I can't say for sure if this is related to the issue mentioned above.

Does anyone have any ideas regarding this?