Okay Michael may come up with a more elegant solution but the issue is the construct_ppurl function. Basically si is not empty so in essence when we construct the url for the thumb si is passed again breaking things.
This condition only exists if cat = 500 and si is in the url so the thumb url gets messed up so my solution is to set si empty when grabbing the thumb url like so
In pp-inc.php around line 537 add the code in bold
Content visible to verified customers only.