Next thing I would do is duplicate the problem mesh, delete history, do a humanIK quick bind and then export that and test it in engine. If that works correctly, then you know the problem has to do with the mesh itself rather than the import/export process. If this is true, then I'd probably start by copy the current skin…
I'm on windows 10 and the installer just gives me a "this app can't run on your pc" message. Admin rights, compatibility mode and developer mode did nothing. Am I missing something? edit: tried the mel script manually and get an error with Maya 2022