Took me a while to figure this out - just done move to new machine and went fully 64-bit. some important pieces of music that I was working on then crashed SONAR when loaded. Eventually worked out it was sfz (new x64 vsti)!
 
Symptoms (other than crashing SONAR) are:
 
sfz (64-bit) loads ok
can load sfz and wav files
try to load sf2 file and it appears to simply ignore it
 
THIS IS OBVIOUSLY BROKEN!
Please Cakewalk, fix this (and up the quality of your releases)
 
Work around is to remove the 64-bit vsti and install the 32-bit version, which seems to work just fine.