Had this happen on a number of occasions. Placed Boost 11 on Master and Plt goes into a freeze. Today I placed Saphire from Waves twice and again same thing both times. Project just freezes. Have to go to Task Manager and end process. No problem there and project starts again without issue. 
Any theories as to why this may be happening? Could it be a conflict with an instance of a vst? Plenty of buffer space and system is not overtaxed. 
 
Haven't had this before...have latest updates etc.
 
Thanks

I have not seen this with Platinum. The only thing that comes to mind is the possibility something is corrupt.  

It's occurred on 2 different tracks. I also just did a test with a clean project and loaded Saphira and it worked okay. Weird one!

I just fired up my system and added Waves Saphire to the master bus of both a new and existing project without any problems.
 
The existing project contained instances of Brainworx Hi-Pass Filter, Maag EQ3, Waves L1, Sonitus Reverb, Waves GEQ and Waves C1.
 
No theories on the cause of your issue (although a conflict sounds likely), but maybe this will help you narrow down the list of potential suspects.

Okay thanks for doing that. I guess I'll have to load the project in Safe Mode with all main vst's out and try adding Saphira or Boost on the master and see if that works okay and assuming so, then add the vst's one by one to try and figure the cause. 

did you add it during playback, or while the engine is stopped?

no, it was stopped...don't make a habit of that.