We had this too. It's even worse when Teamwork saying that the element is locked by another user in team (??? is there any way to do that?) . We normally turn off the file & re-open it, or ask the user who ArchiCAD reported that he locked the element to close & re-open the file.
It's just happented sometimes ago, no more now, dont really know why. It's still a mystery to me!
One suggestion is if possible you should re-share the project. To me it's one of common options (along with deleting local data & re-start & re-join)