Perhaps it's not exactly in the format Freenode's software is expecting?
I and many other mIRC users use Freenode all the time. If this was an issue with Freenode not accepting a particular format, we should be experiencing the same issue :-)
Remember, this issue has been present across many different IRC networks, completely randomly, for as long as IRC has been around. It is a generic connection error that can have many causes, from anti-virus, to servers, to internet providers. It is certainly possible that it could be a client issue but we have never been able to find anything that shows that.
Since, in your case, it is Freenode that is resetting the connection just after a logon attempt, we would need to debug this from Freenode's end.
For all we know, it could be that your internet provider is filtering IRC-related connections because they consider them risky. I recall one user asking their internet provider if they filtered IRC connections and was told that they did not. After much discussion on the forums, debugging, betas, testing, etc. the user contacted them again, escalated the request, and eventually found out that they did filter IRC connections.