Thanks for your bug report.
I did find that I was able to trigger this glitch using Raccoon's speed-test of the various levels of /fupdate.
I ran the script repeatedly but was unable to reproduce the issue. How many times did you need to run it to see the issue?
When I close/reopen mIRC, the problem can go away for a short time, but returns. And eventually restarting mIRC won't make it go away at all, with rebooting being the only way to be able to mark text into the clipboard again.
This seems to indicate that the issue lies outside of mIRC. I cannot think of a reason why something like mark/copy would initially work and then gradually over time work less and less, even after mIRC is restarted, and then work again after the computer is restarted. Also, the fact that mIRC 7.51 can also be affected seems to indicate that this is not a beta issue. Are you running out of memory by any chance? Is another application using up resources?
