Unrealircd 5 made a change today by fixing the reply, but it seems mirc keeps stucks in /list in progress...

The 421 reply you have included in your post is still wrong. Please read my previous post?

I tested the both UnrealIRCd v4 and the latest 5.0.0-alpha3, downloaded today, which use the correct 421 format, as explained in my above post.

Which IRC server are you connecting to that shows the results you are seeing?