Hello there fellow Sonar users.  I'm new to the forum but have been a Sonar user for many years, started with cakewalk when it was just a midi sequencer and now am using Sonar Platinum on Windows 10, my audio interface is a Universal Audio Apollo, ASIO drivers.
Everything had been working fine but recently I bought a few plugins and ran the VST scan in order to use them.  Now, when I insert a plugin in the master bus, the middle seems to disappear and the signal from the right side gets a few dB louder and with a 20-40ms delay, so it sounds like a slapback delay applied to the R.  When I disable the plugin, things go back to normal, but I can't understand what is going on.  Like if I was using a mono plugin in a stereo track or something weird.  However, this doesn't happen with all my plugins, maybe half of the ones I tried (Chandler Curve Bender, Blue Stereo Chorus, Lexicon 224, Pultec Legacy and a few others, some that are clearly stereo and some that used to work regardless of applied to a mono or stereo bus or track).
 
What have I overlooked?!
 
Thank you in advace for your time and replies

Does this only happen on the master bus or any bus? 
 
I'd also never put reverb or chorus on the master bus but that's just me. 

The plugins I bought included the Chandler Curve Bender and that was the plugin I wanted to put in the master bus (I added other plugins then just to see if it was the curve bender causing a weird kind of a M/S effect or not, not that I would have a chorus or reverb permanently on the 2bus)  I also have an external send permanently on the master bus that goes to an SSL stereo bus compressor but that is working fine, no issues with the external send, just with a bunch of plugins.  I noticed it not only happens on the master bus, but on any other bus as well but didn't do too many tests as I had a recording session about to start when noticed this.  If I disable the plugins things go back to normal but when I enable a plugin, the metronome disappears from the middle and goes to the sides with a slight increase in volume and a short delay on the R side.  This had never happened before and I used to apply the UAD Pultec Legacy sometimes in the master bus or the UAD fairchild sometimes but now when I add them I get this issue.  Very strange and I can't seem to put my finger on what is happening.  

What happens if you remove the external send SSL stereo bus compressor and then add the plugins? 

I tried that, but it didn't solve the issue.  I thought it could be that something happened when running the VST scan, maybe it failed to scan an old plugin or something so I started with a new blank project.  I inserted only 1 plugin in the master bus, armed a new mono track and started recording.  Metronome comes out of the L+R, louder on the R by about 2 or 3 dB and with a very short delay on the R side.  I disable the plugin and the metronome goes back to the middle.  Tried this with a bunch of plugins, some I would normally not use in the master bus, but just to see if it was just the UA Chandler Curve Bender, which was not cheap and I really wanted to use on my master bus.  Some plugins gave me the issue, some didn't, I tried about a dozen and I would say that more than half had this issue.

When you say started a new blank project does that include any routing to the SSL? 
 
Have you checked the routing in the audio interface?

Yes, a new blank project, master NOT routed through the SSL.  I checked the routing in the audio interface and don't see anything strange.  I restarted the computer, didn't solve the issue.  Rescanned VST's, didn't solve the issue.

I've tried again today, and am running out of ideas with this issue.  The mono/stereo compatibility of the plugins I think should be ruled out since I'm inserting plugins that are meant to be stereo in the master bus which is stereo.

when scanning the vst, make sure to do a reset in the scan options. just to be sure. also, presumably this is simply a stereo bus, maybe what's happening is the latency reported by the plugin (and perhaps via the UAD hardware?) is off or offset. not a UAD user but perhaps there is a setting to adjust this for UAD? maybe turning off PDC to test? http://forum.cakewalk.com/PDC-override-full-explanation-m2575563.aspx
 

Thank you Glenn.  It's happening not only with the UAD plugins but also with other stereo plugins.  The mono/stereo issue shouldn't be a problem because some plugins should work both in mono and in stereo and some of these are meant to be used in stereo (lexicon reverb, chandler curve bender, blue cat's stereo chorus and others)
 
I'll try turning off PDC and will get back here asap.  Right now I'm about to install the lastet version of the UAD software and crossing my fingers :)