I have heard of some users experiencing this issue however I have never been able to reproduce it and can't think of a reason why this would happen. Normally this connection error indicates that the connection was closed by the IRC server, however this could be due to any combination of your internet connection, anti-virus, firewall, or router. It may be even be related to Windows itself, eg. a recent Windows update.
Since this has only been happening for a week and your copy of mIRC has always been able to connect to Undernet before, this does seem to rule out that it is an mIRC issue, especially since you have installed a clean copy of mIRC and it still does not work. It could be that something has changed on your computer and may very well be coincidental that you are able to connect with a different IRC client.
If you do manage to track it down, please let us know.