cryophonik
There are a few culprits (e.g., Tone2, SpectrumWorx, Eiosis) that I used frequently in Sonar 6-X2 that will either cause a crash or not allow playback, until I find, remove, and reinstantiate the plugin(s).

 
You see, that's a waste of time!  The the plugins remain the same, but if they are removed and re-inserted, then the issue goes away.
 
I just posted another issue, this time regarding the arp and bouncing, in another thread (http://forum.cakewalk.com/FindPost/3388462).  Another example of perhaps an "older project" causing issues.

Just upgraded from Sonar 8 to Sonar Platinum a week ago.  Tried to open my projects and nothing happens, just a blank screen and a forever spinning cursor, so i figured backwards compatibility only considered the X series.  It sounds like some people can get it working however is there a way around this?  right now it's taking hours to save the midi clips and audio clips then reconstructing everything in platinum.  

I would contact CW Ryan. There is no reason I know of for not being able to open an older project. 

John
I would contact CW Ryan. There is no reason I know of for not being able to open an older project. 

thanks will do!  I haven't tried saving them as a .CWB yet so maybe that's the issue but .CWP's definitely do not seem to be working at all for me for some reason.  

Don't save as a CWB file whatever you do. That is adding insult to injury. 
 
That is not a cure for a problem it can cause a problem.