I'm on a DP 800 with Logic 5.3 and all works fine here. Do you have any plugs in the old song that might not be DP compatible? NI has quite a few as well as all of the Pluggo plugs.
You probably need the latest version of Apple CPU plugins. It's an extension. If you've upgraded your system from an old version, you probably don't have it enabled.
Hopefully this is the problem. Get a copy of the mac os 9.2.2 installation cd, and copy the extension from the cd to your hard drive.