welcome to the forums
does it crash even when you click on custom battle immediately after you start the game? without clicking on the campaign button? because the crash when switching between custom battle and campaign modes is a known bug, that i don't know how to fix
edit: i just realised you are saying that your game crashes in campain battles too.. thats odd
i don't know what might be the problem here, since it is working with everybody else
but you can try to replace the attatched file, maybe your download has been corrupted
just to be sure, when you click on custom battle does gondor have TONS of units? because if they don't, you did not install this mod correctly