One possible reason for this type of issue is that you are using scripts or addons, usually DLLs, that need to be updated.
If you type the following command into the status window in mIRC and press the enter key, what are the results?
//echo -a $os $version $beta $md5($mircexe,2) $file($mircexe).sig $alias(0) $script(0) $dll(0) $com(0)
Another possible reason is anti-virus software that is interfering with mIRC. If you add an exception to mIRC in your anti-virus software, does that resolve the issue?
Finally, you could try re-installing mIRC v7.45 using a full install, which will overwrite all of your existing settings, to see if that helps resolve the issue.