In a nutshell:
 
Out of the blue, my RME 9632 + Sonar 8.5 + Kontakt 5 started giving me the left channel audio out ONLY on all my files.  No right channel at all.
 
If I switch to Reaper, both channels working again!
 
Now... strangely.... if I use my old FTULTRA , Sonar 8.5 is happy again ... both channels working.
 
I must have tripped some setting in Sonar that tells RME to kill the right channel, but I can't find that setting. 
Tried a re-install of Sonar 8.5, but no change.  
 
 

This post by scook about resetting Sonar's defaults might help:
 
http://forum.cakewalk.com/FindPost/2868188
 

Jon's advice is good.
I'd start with Aud.ini and let Sonar repopulate it.

Yup. A hardcore reinstall is probably the way to go.

Doing it.

It's not just Kontakt.
 
I had once, and again two days ago Clap Machine vsti synth go all right channel on me.
The solution thankfully was to save the calmly save the file, restart Sonar and reload the file.
 
I also have an RME 9632 but I doubt the output driver is at fault here.
 
There are little regression bugs surrounding the changes to SONAR they were making in the last year. But we have to just live with them now so yeah.

Yup... not an RME issue.   Just ran Cake Home edition, to check.   No problem there.   A Sonar 8.5 wrinkle.   Will completely erase it and reinstall.