Clean out routing?

I put it down to incompatible routing chains. A few times my ProChannel has crashed which I think was one of the PC4K compressors. So that might be a cause. Anyway, if you have any recovery files in the same directory as the your project, check the timestamp and rename it by deleting the extra info after the cwp and it becomes a project file.
 

interpolated
I put it down to incompatible routing chains. A few times my ProChannel has crashed which I think was one of the PC4K compressors. So that might be a cause. Anyway, if you have any recovery files in the same directory as the your project, check the timestamp and rename it by deleting the extra info after the cwp and it becomes a project file.
 

I did a test and the crash happens when I select any type of side-chain.  Seems to me the more you work on a project the more garbage is built up in the background like a Web browser. 
Also, when I realized I wanted to remix this project, I went into the mix scene memory and cleared all previous mixes and reset the project to reset state.  No, effects, panning, routing, etc. 

I don't think its a cache issue, When you said... "Sonar crashing when assigning a side-chain"
Very important: What plug in are you side chaining ?
 
What plug ins are you using and what plug in are you using to side chain ?
I would do a test and remove the plug ins. If you don't crash, add plug ins one at a time.
you might find an offending plug in that is not playing nicely with your project.

John
I don't know of a way to clear routing. Never thought about it before. It might be a good feature request though.
 
One thing you could try is copy the old project to a new project track by track and so on. Save it to a new project folder.

I tried to export the individual raw tracks.  The wav files sounded like noise when I imported them into a brand new project.  However, when I click on a file in the Windows folder, Windows Groove player plays the track just fine.  Unbelievable!  No winning.  I'm back to the original project.  I will forgo side-chaining anything.

John
Something is amiss. None of the issues you are reporting should be happening. I would look at your overall setup making sure all your drivers are up to date and that Windows is also up to date.  Many think that allowing the updates to Windows is a bad idea. I think that is a wrong headed notion. Those updates are to fix issues and add further security.  I'm not saying you don't update but if you don't then that may be the problem. 

John,
My system updates all of the time.  One of the things I hate about Windows 10 is that it will download updates in the background while working on a project.  In Windows 7 I had it set for notify so I could update between working on mixes.
 
My computer was built and is used for nothing else except Sonar and programs that work with Sonar.
 
Yes, I'll see if maybe my Focusrite or any other driver is up to date.
 
But think about it; I'm able to close the problem project, open a new one and side chain with no problem.  No exiting of Sonar, no reboot.  Yet, if I delete every single plugin from the problem project (which I did), reinsert two plugins, and try to side chain, the program crashes.  There is no question in my mind that there is something in the background of Sonar that's already been corrupted and can not be repaired by resetting the mix to zero (no plugs).  We can delete picture cache.  Why can't we delete all background remnants from a wiped mix?  
 
Other thoughts - Maybe there is an offending track?  I haven't tried deleting tracks.

jmasno5

Update

If I delete all of the Summing Buses (drums, vocals, guitar, etc.)  side chaining works!

Can you delete one at a time to see if it's a specific bus, or does it require all/multiple?