I can add an AD2 instrument track and mess with it, record some MIDI into it and it's all good until I save and exit. The next time I try to load the project, Platinum instantly locks with a sort of "glazed-over" white screen effect. Anyone have any ideas what's going on here?
 

Win 7 64 pro
Intel Core i7-4770 3.4GHz
24gb ram
Nvidia Geforce GT 635
Onboard Realtek HD Audio (don't laugh, it's surprisingly not terrible!)
USB Audio Interface (USB standard compliant Behringer mixer without specific drivers as per manufacturer)
MIDISport 4x4 Anniversary Edition

Hi. Could it be that your soundcard is busy with any windows instances?
I would strongly suggest that you get an Audio Interface (cheap or not) that supports ASIO.
Best of luck.

Well, I did have to set the soundcard properties to "exclusive mode" in order for Platinum to stop claiming it was "unavailable" (Pro wasn't a problem, but for some reason Platinum isn't happy unless I set this).
 
Weird thing is that AD2 works fine when I add it and record MIDI into it, it's only when I try to reload the project later that this happens.

Could you post a screenshot of your issue (if you have any still)?

I'll see if I can figure out how to post images, but there's really nothing to see. There's no error message, it's just the blank Sonar interface with kind of a translucent white overlay effect over the entire screen, greying everything out (like maybe it's trying, but failing to pop up a window or something). The cursor is in that spinning "working on it" mode and everything will stay like this until I click somewhere, then the standard windows "this program has stopped responding" box pops up giving me the option to close it or let Windows pretend to search the net for a solution and then close it. 

Please go to the XLN update and make sure you are up to date.
Try performing a VST reset:
https://www.cakewalk.com/Support/Knowledge-Base/2007013315/SONAR-X3-Full-VST-Rescan-Recommended-for-Upgraders
 
Try running Sonar as administrator.
Please make sure windows update is up to date.
Make sure your audio interface drivers and firmware is up to date.
Disable all your audio interfaces in control panel -> Sounds.
Disable your antivirus. 
 
... and report back if no joy.
 
Thanks.

Working for me. I just opened a test project that had 3 MIDI tracks feeding a single instance of Addictive Keys. I added an Addictive Drums 2 instrument track, rerouted the MIDI tracks to it, played it (piano MIDI played through AD2 sounded interesting; ha!), saved the project, exited SPlat, restarted SPlat, and opened the saved project no problemo.
 
No help here. Sorry.

Would also have been useful to know about this..
http://forum.cakewalk.com...spx?m=3251246&fp=1

...advice remains the same though..

Sorry, it didn't occur to me that this issue might be related to the other one. Yeah, as several suggested, it looks like ASIO4ALL was the culprit. Uninstalled it and now the projects open and playback normally. Thanks again for all the help everyone! :)