It just happened all of a sudden, i can't change slots or assaign any new members to it
Yes they are combatants
Yes its its in every screen on every location i've tried
Edit: I managed to add them in the combat party window you get before special battles like in quests, but battle group still doesnt work

Edit2: I  was able to add jean to the party, she's the named character that worked

Edit3: Right so now it works but the game itself is broken, can't get into the mansion without loading a save file

I also just encountered this same issue today. Can't add characters or move them.  Nothing shows up error-wise in the log when I try to move someone, it just blocks it with that "no clicking" or "not allowed" symbol.

Have the same glitch on one of my old saves. I found out that bought new characters after I boot up the save aren’t effected, just old ones