Forums mIRC Help agentctl.dll missing: How severe is this?

This issue will be fixed in the next mIRC release. That said, I would not recommend using older versions of mIRC unless you absolutely have to - new releases of mIRC fix a whole range of issues, including security-related issues. By using older versions, you are leaving yourself open to exploits. Is there some reason that you prefer 6.12 over 6.35? Is there something specific about the latest version that you do not like? If it's something simple, let me know, maybe I can improve it for the next version.