Hi aqua girl, with what we know so far it's definitely a firmware issue on a few players and that means a replacement DVD will not make a difference. We've only received a few Stacked Upper Body DVDs back from people that reported the same issue as you have, but in every case, the DVDs that were sent back and we tested all have played fine so far. Since thousands of people have successfully played the Stacked Upper Body DVD and we have successfully tested a few of the DVDs that were sent back, it pretty much rules out any other possibility besides this being a firmware issue. I still want to see some more returned discs and get some reports back from customers that were sent a replacement disc before closing the book on this though. But so far everyone that has reported the same issue as you, and who have contacted us, have also reported that they can play the Stacked Upper DVD on other players. We've also expanded our testing to 15 different DVD players and so far we have not had any issues.
I did notice that someone else reported that they have a SONY 5 Discs player like you do (not sure if it is the same model as yours) and they could play Stacked Upper Body just fine. If they do have the same player as you it means they probably have a different version of firmware on their player as you do and is why they can play the same DVD that you can't. This means there may be a firmware update from SONY available that will solve your issue. However, there is no way to know if the newer or older firmware is the issue. But you may want to contact SONY and see if they have a link to a firmware update that you could download that may solve your issue.
I did notice that someone else reported that they have a SONY 5 Discs player like you do (not sure if it is the same model as yours) and they could play Stacked Upper Body just fine. If they do have the same player as you it means they probably have a different version of firmware on their player as you do and is why they can play the same DVD that you can't. This means there may be a firmware update from SONY available that will solve your issue. However, there is no way to know if the newer or older firmware is the issue. But you may want to contact SONY and see if they have a link to a firmware update that you could download that may solve your issue.