When I use /List on Freenode I get this:

-brown.freenode.net- Your client is buggy, it sends LIST <10000 even though we don't advertise ELIST=U in 005. Please upgrade or contact the developers.

That's a server problem, not a mIRC one.
Try on another network and I'm sure it will work.

Try this first
/quote list
And see if it still yells, if it does then its the server itself, otherwise your mirc is sending out a command to look for rooms with less then 1000 people