Apparently the limitation for grimstone masks is not just to Ed (though he has his own message) but to all non-standard classes? Vampyres certainly get a message saying ""You can't use this item as a non-standard class. Sorry.".
Would you consider modifying the version checking to either stop checking a URL at the forum or possibly munging an old style URL to the correct one? (That was possible on another vBulletin to XenForo upgrade and I can help dig if you need it).