Changed label and description, and got an error because of the new label (?) and previous description (?) had a name clash with another (his father) item. I would guess the code does not check for a change of both the label and description at the same time.
Description
Description
Related Objects
Related Objects
Event Timeline
Comment Actions
This is due to the fact that such an edit is actually split in to.
First an edit is made to the label and saved, and then to the description
Comment Actions
Seems like Nikkis conclusion is correct. I added an alias and afterward I could edit the label.
This bug is thus a duplicate of both the bugs Nikki pointed out.