Thanks for your help in testing the new betas and tracking this down.
It looks like the issue is due to the recent changes in Control+Break. Control+Break was changed so that it is detected only when mIRC is active. This required a more complex combination of API calls.
So far, I have not been able to reproduce the issue in Windows 7 or Windows 10, so it looks like it triggers only in certain contexts, which is why it is intermittent.
I have made a change to Control+Break, to how the APIs are called, that seems to resolve the issue for you.
This fix will be in the next beta.