Some certificates include more information than others about the host name of the server. In such cases, mIRC will try to extract more information from the certificate to verify the host name, which is probably something that other IRC clients don't do.

I have reproduced the problem and mIRC crashes when I try to connect via SSL to, however as far as I can tell from the source code mIRC isn't actually doing anything wrong. So it may be that either the OpenSSL library has a bug, or the SSL certificate provided by the IRC server is malformed in some way.

For the next version of mIRC, I have disabled the code that does the extra host name checks.