There is definitely a bug, at least in the demo version.
I downloaded the Demo version "TSE_X50_v2.0_x64_demo.zip" on Feb 14, 2014. I am on Windows 7 64-bit.
There are quite a few ways to get Reaper and TSE x50 to crash. The easiest way for me is to load the x50 on one guitar track, and then on the mixer window, drag and drop this instance on to the second guitar track. Sometimes it will work, sometimes it will crash. So if it works, delete the second instance and drag and drop again until you get it to crash.
As far as I can tell, it is completely random. I leave the x50 with default settings too (changing them seems to have no effect on the probabilities of it crashing).
If you happen to save a session with two instances loaded and close it, when you reopen it, sometimes the session will open. If it crashes, double-click the session again until it opens.
Problem signature:
Problem Event Name: APPCRASH
Application Name: reaper.exe
Application Version: 4.5.9.0
Application Timestamp: 52d9a146
Fault Module Name: TSE X50 x64 DEMO.dll
Fault Module Version: 0.0.0.0
Fault Module Timestamp: 52fed79d
Exception Code: c0000005
Exception Offset: 00000000000810b3
OS Version: 6.1.7601.2.1.0.256.48
Locale ID: 1033
Additional Information 1: 587d
Additional Information 2: 587d8adb89f11a36df3331a168e75b10
Additional Information 3: 448c
Additional Information 4: 448c421a10d3dd976212f4da4e4f752d
I should also say that the plugin sounds really good, and I'm afraid the discount offer will expire before this gets fixed. So I am willing to help!!