To reproduce the issue follow the steps:
1. Go to Windows tab and open REX extension
2. Play scene
3. Write any expression into REX window (etc. "x=1+1") and evaluate
4. At this point you have REX up and running with out any issues
5. Stop scene that was play
6. Play scene again
7. At this point Unity freezes.
It is not consistent, might take a few tries before the issue occurs.

How to get stacktrace:
8. While Unity is frozen, open up and attach debugger to Unity
9. Hit "Break" button, which should break the current execution
10. AT this point Unity throws the crash report window

Reproduced in 5.4.0b22, 5.3.4p2