Hey Richard,
Glad to hear you guys are longtime PB enthusiasts. Definitely wasn't saying that you guys didn't know how to get the stringparm, I was just trying to say that something else may be happening now that is messing with the value. Often times with newer versions of PB, the timing of events can change, so the message object stringparm may have been fine where you are accessing it in the prior version, but now something may be stepping on it. I attached a simple quick app I wrote that uses the message stringparm and opensheetwithparm and it is working. I would definitely check any ancestor code in the sheet's open event to see what else may be happening before you access the message.stringparm.
Good luck,
Kevin