Sounds like you've build it using advanced, it will change back to humanfriendly, but you'd need to review all the url links I think. Plus you'd probably take a hit on the SEO for a couple of weeks.
Trouble is if the advanced url re-writer is not passing back the correct prams to the module, we can't do anything!! I'm unsure, but maybe in the setting of the advanced url re-writer you could tell it to ignore that specific pattern that is passed back from paypal. Maybe you can test for the "stg" param and then exclude the urls with that param from being re-written?
To check if the checkout module is indeed getting the response from paypal, look at the LogFile folder under the portal root, you'll see a log file, id the response is taken by the checkout module then you should see a line that says "IPN=...". See what's in that!
Trouble is if the advanced url re-writer is not passing back the correct prams to the module, we can't do anything!! I'm unsure, but maybe in the setting of the advanced url re-writer you could tell it to ignore that specific pattern that is passed back from paypal. Maybe you can test for the "stg" param and then exclude the urls with that param from being re-written?
To check if the checkout module is indeed getting the response from paypal, look at the LogFile folder under the portal root, you'll see a log file, id the response is taken by the checkout module then you should see a line that says "IPN=...". See what's in that!