Which version of mIRC were you using before upgrading? Are you using scripts?
If you install a clean copy of mIRC, using the portable option in the installer, in a new, empty folder, such as your Windows documents folder, and run that copy of mIRC, do you see the same issue?
I was using version 7.49 using scripts and normal version( not portable), I've installed in an different folder(E:\mIRC).
I've solved my problem here.
I tried a clean installation in a different path, worked fine.
Then I removed this installation and reinstall over my old installation and everything worked perfect.
ps: I'm not using the portable option.
Thank you for your attention.