That's a strange one-  I always use the "Instert Synth" menu and have never seen it.  On the other hand I've seen some strange bugs that other people never seem to encounter. 

Shows how hard it is to write software that runs on various combinations of computer hardware, operating system, 3rd party plugs, audio hardware, and interfaces.

Lately, I've also been seeing this occasionally. If I do a VST scan that seems to fix it. I haven't figured out what makes it happen. I have X1 set to scan on startup.

I've never had that problem and I use Insert-Softsynth and Browser often. 

Are you perhaps overclocking your CPU? 
You may also have some corruption on your hard drive. You should probably run a CHKDSK.