Thank you for your post. If you can please type the following into the status window in mIRC:
//echo -a $os $version $beta $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0)
and post your results here, this will help in tracking down the issue.
Please also try the following:
Install a clean copy of mIRC into a new, empty folder in your Windows Documents folder using the "portable" option in the installer. If you run mIRC from this folder, do you see the same issue?
Are you using anti-virus/security software? If you disable and close it down completely and then run mIRC, do you see the same issue?