Under certain conditions, when you click Connect in the Connect dialog box, mIRC will attempt to connect to an IRC server that doesn't support SSL using SSL, which obviously will fail. It also ends up trying the wrong port.
These are the steps I can reproduce this 100% of the time:
1. Create an entry in the Servers dialog for a non-SSL server, such as what I use, irc.speedrunslive.com port 6667.
2. Create another entry in the Servers dialog, but this time for an SSL-supporting server, such as what I use, irc.teksavvy.ca port +6697 (note + sign).
3. Exit mIRC.
4. Start mIRC.
5. Using the Servers then Connect dialog, connect to the SSL server (irc.teksavvy.com +6697).
6. After it logs in, spawn a second mIRC process, such as by middle-clicking the mIRC taskbar icon if on Windows 7/8/8.1/10.
7. Select and connect to the non-SSL server (irc.speedrunslive.com 6667) using the Servers and Connect dialog boxes.
The second copy of mIRC will attempt to connect to the non-SSL server using SSL incorrectly, even though 6667 is specified in the server configuration.
The /server command seems unaffected by this bug.