Sorry couldn't open it in PanelBuilder32 either. It said it was going to convert it but then said it could not open it. PB32 is the oldest I have...good luck
When the application file got too big for RAM, the old PanelView 550 and 900 had an option to let you strip out the data that was required to upload and re-create the PanelBuilder Application file.
That might be what's happened here. I don't know of any way around it.