I'm having exactly this problem.
Is the solution posted somewhere? When I changed the line WA_simlib.php file:
return (bin2hex (mhash(MHASH_MD5, $data, $key)));
return (bin2hex (hash(MD5, $data, $key)));
I did get a value back in the error message from Chase Paymentech. But when I compared it to the one generated on their online "tool" to verify it, they were different. MD5 is chosen as on the hosting site.