And no, no sandbox required.
As for diagnosing your problem, start by running warhead without the mod and see if the problem persists. Also check access righ`ts for crysis folder as well as mwll folder. You should set access rights for "everyone" to "full control" crysis directory. Also, try running crysis as an adminisreator. You can also try to make one more complete uninstall(again, manually deleting crysis folder after automatic uninstallation if it remains). I know, it sounds as a stupid advice but, judging by forum posts, it helps sometimes.