Thanks for testing this out. In RFC1459, the 421 numeric is defined as:
:host 421 command :reason
The "command" needs to be specified so that a client knows which command caused the issue. In your post, it looks like UnrealIRCd is using "nickname" in place of "command"?
That said, numeric 421 as returned on almost all IRC networks changed a long time ago to this:
:host 421 nickname command :reason
This is the format that mIRC is expecting.
Update: I just tested this on UnrealIRCd v4 and v5 and they are returning:
:irc.foonet.com 421 nickname COMMAND :Unknown command
which is the expected format. Are you sure you typed the debug results in your above post correctly?
I also tested disabling the "list" command in UnrealIRCd v5 and it returned the expected 421 format which was parsed by mIRC and enabled /list again.