Good advice Bob!

Thanks a lot for your answer and experiences Bob.

I've been having this issue as well. The cause in my case was that SONAR.exe was still running in the background, even though the window was closed, and one would normally think Sonar is indeed shutdown. I'm having to go into Task Manager and manually kill SONAR.exe almost every time now.

This can be caused by sonar not releasing sound card drivers properly. Probably the fault of the sound card drivers. Sometimes you cannot end it from task manager, but instead of restarting you can log out and log back in again. Users have reported success with that.

+1 on the drivers, double check to see if you have the latest sound card drivers installed.
 
I had this with my Delta 1010 and no M-Audio drivers would resolve this issue, since changing Sound Cards I havent had an issue.

In my case, I can end it in Task Manager every time. No changes in drivers, or apps, but I did notice this more after re-installing iLok drivers, EastWest Quantum Spaces, and Waves IR-1.

MIDI controller drivers can cause this issue as well...
(Korg USB MIDI drivers used to do this)

brconflict
I've been having this issue as well. The cause in my case was that SONAR.exe was still running in the background, even though the window was closed, and one would normally think Sonar is indeed shutdown. I'm having to go into Task Manager and manually kill SONAR.exe almost every time now.

Check to see if BitBridge is running as well, and if so, check for any 32-bit plugins in the failing project, as one or more may be causing this issue.
 
Bob Bone
 

Yes, I agree with Bob. What I did was loading a new and empty project in X2 (choose for the upper possibility in new project), and closed X2 after that. In that case X2 closed really and was no longer in the taskmanager. I could start it over and over again in this way. So I was sure it was a matter of the plugins I use! Still finding out which plugin is the cause, as Bob says it should be a 32 bit plugin.