The only similar issue I've seen lately is
which was reported only in Win7 in v7.62, and 7.63 seemed to fix it. Which was the most recent version where you did not have this problem?

One thing you can check is to confirm that your 7.63's installer and mirc.exe itself have valid digital signatures. Under 7.63 I get the following result:

//echo -a $version $file($mircexe).sig $beta $hmac($mircexe,2)

7.63 ok 05ab7ff1607621b3a990ebcb402d86ea79cee379