I had this in X2 as well - and not certain what the exact cause is. Either way, I think the loop browser is generally a little unstable. One thing that made it noticeably worse was playing samples in loop mode, at the same tempo, while the song was looping and then switching samples. That seemed to get Sonar's processing in a bind and led to the semi-transparent white screen.
Now I just preview loops in isolation and drag them into the track pane to preview with the rest of the track. Takes considerably longer of course, but has so far avoided crashes.
In fact, X2 does seem susceptible to crashing if you make too many changes during playback (certainly compared with 8.5). For that reason I tend to apply new inserts, sends, new busses or pretty much anything similar while tranposrt is stopped, and I always make a save before!