I figured what happened when I could reproduce it (with a lot of troubles) :
When I lose, my vassal, the king of England, actually gets the targeted realm of the crusade, which is... his kingdom. He is catholic unlike me, who is cathar, and that must be why he is the one who can get it.
If the Kingdom is mine, the Pope gets it instead and it becomes independant from my empire.
I would guess this is not how it should be (he should become independant or the catholic crusade couldn't be called on a kingdom who is already catholic).
I do have a save of it (you just have to surrender to see it), but I don't know if I'm allowed to put external links to uploader since I have an error when I try to upload it here.
For the checksum, I have KEXH when entering the game and JEXH if I resign with my save, is that what you needed?
Hope this helps.
Actually, that should be enough information to go on. The edge case here is the Catholic owner of the kingdom also being your vassal, which I doubt the CB code accounts for. If I don't find an answer in there, I'll ask you for the save -- but that sounds right to me. I'll fix it for the coming update.