In the favourites section of Mail (in screenshot at location (1)), a folder is greyed out. It cannot be selected, also nothing happens if I hover over it. What could be the reason? Please note, the folder was before in another section ("On my Mac", (2)), I probably moved it to the favourites section by accident. Also I cannot drag it back to any of the non-favourite sections.
Searching for this kind of problem was unfortunately not successful.