Couple of things - maybe you mean you 'can't' open an app with CB21 ? If so, try opening with CTD1.5.1 or CTD3.0., or try removing the service pack temporarily. Maybe the code is 16bit, so you should try using SqlWindows v5.x ?
If you can open it but cant run it, try saving the file as a .apt (text file) and then re-open the .apt file. Any text errors will either be highlighted or fixed.