Found the error: 

What was causing the "...An unanticipated error occurred while processing..." problem was that I had set the "hostedPaymentReturnOptions" to a filename plus GET variables. So, setting the cancelUrl to file.php?msg=1  was ok, but setting it to file.php?msg=1&id=5 made it die.  I still obtained the token just fine, but when going to the payment page it errored, without a meaningful message.

Thanks for your help.  Moving on with testing...