After updating, completed tasks show up under "To do" again

[ElisabethNiederbacher]

I connected Open Tasks to an external program (Rentman) using the CalDav synchronization. The tasks do show up in OpenTasks and when I complete a task, the date the task was completed on also shows up in the external program.

The problem is, when I update OpenTasks (see screenshot), the completed task shows up as "To do" again. In the external program it still shows up as completed. Can somebody help me figure out why?