I was working with r53 for a bit just now and unfortunately it happened again. At least I have a hunch now what triggers it.
When I make changes to a project, then close Construct and click on "Don't save" in the appearing dialog, the exe seems to remain and can't be terminated anymore. Restarting Construct 2 will work, but one can't preview anymore as mentioned.
Not 100% sure that's what causes it for me, but at least it just happened that way.