Sonar 8.5 CWP files

There shouldn't be a problem opening old projects in new releases. Is it just one project or all projects that won't open? Are you opening from within Professional, or by file association, clicking the .CWP file? Kind of sounds like it's trying to open in 8,5, and that installation is still corrupted. The Professional installation will be totally independent from it.
 
if it's just one project, there may be some plugin in it that's causing trouble. Try opening in Safe Mode by holding Shift as you launch the project to get options to selectively load plugins.
 

  Weird error. You're welcome to take it from here. 

Please tell me you have more than one copy of your project files. You should have them backed up to an external drive or even better 2 or 3 different locations. It's a lot of work to loose for what takes a few minutes to back up safely. 
Somehow I get the feeling you imaged your old drive and this is why you getting the messages. 
 
Best practice. 
Always back your projects up to an external or second data drive after any serious work has happened. I do this at the end of each day before shut down. 
Every so often back those up to a third location and date the folder. I even have storage off site. 
 
When ever I build a new computer I of course by a new main OS drive. 
If I upgrade an old computer I buy a new OS drive. Drives are cheap. 
The old drives are put on a shelf or added as data drives. 
I have never had any issues with old Cakewalk files as they are all stored inside folders and all audio, midi CWP and everything else is kept in these folders. Simpley drag and drop those folders to and from drives and they will always be 100%.