Many networks don't allow _ as the first character in ident. The error you posted clearly shows that the network is the one not allowing you to connect using that username. It has nothing to do with mIRC or any sort of bug. As stated, you can change your ident to not use the _ as the first character and then you can talk to the people in #help to find out why they have that policy. It is unlikely that they will change their policy on usernames. You may not have issues on other networks because other networks may not have the same policy.
As far as registration, you are registering a NICK, not a USERNAME (aka IDENT). Nicks may not have the same limitation.
