The only other cause I can think of is anti-virus/firewall software - sometimes when they update their signature files they may start detecting valid files as trojans/viruses (this is called a false-positive detection). Perhaps your anti-virus/firewall software is forcefully closing mIRC when it runs because it thinks it is a trojan/virus, which results in this issue.
You can try closing down your anti-virus/firewall software (although note that sometimes they still partially run in the background even if you close them down) and then running mIRC.
You can also try explicitly adding mirc.exe to your anti-virus/firewall exceptions list.
