This used to happen to me all the time. The way I solved it was to shut down Sonar X3 in this order. Instead of just pressing the "X" in the upper-right corner, go to File->Close first. Then wait a few seconds (you'll see the borders of X3 change color slightly indicating it's not doing any work) and then exit out of X3 entirely, either by going to Menu->Exit or just pressing the "X" icon. 

If you are already doing this and still getting this error, then I don't have any advice :P

Good advise here.

Here is a suggestion.

1) Uninstall mix control (whether on latest or not).
2) Restart windows
3) Install latest mix control from focusrite's website.
4) Perform reset and restore default settings in mix control.
5) Make sure windows update is up to date (most issues occur from not updating, not the other way around), and your chipset drivers are on the latest (check Intel driver update or and equivalent).

Ta

Oh I didn't catch that, we do both have Focusrite and Mix Control. And as another point, I don't often open mix control. You don't need to unless you are adjusting monitor mixes or the input line/instrument toggles. And this was a time where I had it open and over on my second monitor. Could just be you need to close mix control and all would be good.
I don't see the point of re installing the drivers if they are the correct ones. I just checked and mine are up to date still. All that might do is force a re set to the factory default which you can do from the menu. Try that instead.  

+1 to johnnyV, when bringing SONAR online, interfaces/controllers should be online first (and seen by Windows). When closing, the reverse is true... Project, SONAR, then interface/controllers last. SONAR tries to respond to changes in interface/controllers on the fly, but takes a few to several seconds, so is best not to "stress test" that while also doing others things.

johnnyV
I don't see the point of re installing the drivers if they are the correct ones. I just checked and mine are up to date still. All that might do is force a re set to the factory default which you can do from the menu. Try that instead.  

Sorry but you are mistaken, otherwise I would not have recommended it and I would not have stated "whether on latest or not".

I've had issues at least twice where cleanly reinstalling + reset fixed the issue. The problem is that upgrading from earlier mix control versions sometimes corrupts settings, not only at the hardware end but at the software end as well, and a clean install+full reset sorts it out. I've also recommended it to others in the past and this has fixed their issue, I've also have been in contact with Focusrite about it a while back. So what I state is tried and tested (and seems to be known about from Focusrite).

I strongly recommend carrying out the procedure I stated in the previous post in full to see if it helps. Full reset inc hardware.

Nowadays when I upgrade mix control I always perform a clean reinstall+reset as I don't trust the product to upgrade itself correctly.

Thankyou..

To which I would add...this doesn't necessarily apply only to Focusrite.

But this makes absolutely no sense to my pea sized brain, I do believe your right in the case of software that seems to be not working, But my version of mix control seems to work perfectly fine and for about 8 months now every day I use it without issues. And it is the only version I have ever used since day one. 
 
I just completely re did my system and when I did I had the latest version of mix control so that's what gets installed. It is  1.8   which I just checked and it is still the most up to date version. 
 
So why  would  re installing something that is working perfectly fine change anything? Software should not be that flaky. Sure if it was not working right because of a possible glitch in the download. But even that seems far fetched as most corrupted download will throw errors when installing, There's code to check these things built into the installer, right?  
 
I will agree that sometimes a complete fresh install of everything is sometimes faster than endless troubleshooting and registry hacks.. that's for sure. But in my case I just installed it a few weeks ago. It could help the OP however. 
 
I have no clue why the same thing happened to both of us, and nobody else here does either, It's either a coincidence that we both have Focusrite interfaces and some combination of whacking on buttons and stuff before closing Sonar caused a random glitch. I cannot re create it, I tried. So it's was just one of those daily weirdness you will always experience when working with these new fangled computer systems.. I'm hoping my new build will lessen the daily weirdness by a factor of 9. 
 

johnnyV
But this makes absolutely no sense to my pea sized brain, I do believe your right in the case of software that seems to be not working, But my version of mix control seems to work perfectly fine and for about 8 months now every day I use it without issues. And it is the only version I have ever used since day one. 
 
I just completely re did my system and when I did I had the latest version of mix control so that's what gets installed. It is  1.8   which I just checked and it is still the most up to date version. 
 
So why  would  re installing something that is working perfectly fine change anything? Software should not be that flaky. Sure if it was not working right because of a possible glitch in the download. But even that seems far fetched as most corrupted download will throw errors when installing, There's code to check these things built into the installer, right?  
 
I will agree that sometimes a complete fresh install of everything is sometimes faster than endless troubleshooting and registry hacks.. that's for sure. But in my case I just installed it a few weeks ago. It could help the OP however. 
 

Because mix control comes with buggy upgrade scripts effecting some scenarios. Includes perhaps mishandling file settings, registry and firmware... Best clear it and start again as people have experienced it. Reinstall takes 15 mins max and is generally simple so there is no reason not to recommend it.Ta.