I've seen this happening for years. To my knowledge it occurs whenever an SSL connection is ended with /disconnect rather than /quit. Quitting mIRC in the ways you described sends a /disconnect, so that should be why you (or others) are seeing that "Input/output error" message when you are terminating an SSL connection.
This issue has been around as long as I can remember, probably ever since mIRC got built in SSL support. I reported it many years ago, but unfortunately never received any replies.
I'm not sure if mIRC or OpenSSL is to blame for this, but I do know that back in the days before mIRC had SSL support, I was using Stunnel and StunTour for SSL connections, and with at least one of them, I didn't see this issue when using /disconnect.
