What UI error? Switching from basejka to OpenJK means you also have to use the other japlus folder. So instead of GameData/japlus it'll probably be /Documents/My Games/OpenJK/japlus. Make sure everything is in that japlus folder instead of the base one. OpenJK basically allows you to have a second base folder, both of which are used when launching the game. But anything in GameData is not used, but rather the one used in the OpenJK folder are. Hopefully that makes sense.