I'm not sure why or how, but after checking my mod list, I found that this particular mod is causing the game UI to behave very strangely. It creates empty black squares, making it impossible to assign an ideology role to a particular pawn.

I also can't use the Character Editor because when I try to assign a trait, it creates a black square again. It's a shame because I really love this mod... And I don't have any incompatible mods installed.