internal error : 500 when call cwa service via error MainCommandHandler.ashx

Today tried to call cwa method to verify user online status. logon successfully using "Forms", but call Initiate Session gives me 500 internal server error. this is because MainCommandHandler.ashx does not exist on server !! must use AuthMainCommandHandler.ashx, terrible! when I modified, it passed! but call another method shows : error code="18100" reason="Session not found"