mIRC - Software Caused Connection Abort

[Damaeus]

I've been having a problem staying connected to UnderNet. I repeatedly
get "[10053] Software caused connection abort" in the status window, but
so far, only from UnderNet. I can be connected to EFNet in a different
window and not lose my connection there, nor even when I'm connected to
freenode.

One day, someone in a channel I was chatting in contacted me and said he
wanted to test something with my permission, thinking that someone may be
sending me a string of characters via a private message that I would never
actually see, but which would cause this [10053] to disconnect me, and it
did! The problem is, I have no idea how to guard against allowing this
code to disconnect me.

At the time, I tried upgrading my old Linksys router to the latest
firmware because he said that would fix the problem, but it didn't. Now
I've got a new higher-end Netgear N600 WNDR3700 dual band gigabit router
and I still have the same problem. My computer is on a wired connection,
not wireless.

What should I do? This thing is eating my grits and driving me insane.

Damaeus