Error: The material status was NOT changed to complete, status=C. Please report this to the developers

what does that means?

This has something to do with the fact that you clicked the “I’m all done” button but you hadn’t read or listened to the item. This shouldn’t have happened and likely won’t happen again so please ignore it.

In fact, we are going to remove this button and the Complete status soon because there is no point in having them. If you are finished with an item, the best thing to do is to archive it.