Thanks to the posters reporting this issue.
We managed to track down the issue to mIRC checking if there is an external OpenSSL library and loading it to see if it is a newer version than the internal OpenSSL library. In the past, this has never been an issue, however it appears that, in this case, the external OpenSSL DLLs found in the system PATH are compiled in a way that is causing an MSVCRT conflict. If the user sets mirc.ini [ssl] load=2, this prevents mIRC from checking for newer external OpenSSL libraries and resolves the issue.
The next version of mIRC will be changed so that, by default, it will not check for newer external OpenSSL libraries.