A few things:
1. Undernet's "Ident broken or disabled, to continue to connect you must type /QUOTE PASS #####" server challenges can be triggered when your port 113 (identd) is stealth firewalled. Try properly port-forwarding TCP port 113 to your computer, and opening TCP port 113 in your software firewall. (Whether you additionally enable mIRC's identd daemon or not, though, doesn't matter.) Also, these challenges can be triggered if your identd daemon is enabled, and if its "User ID" field is set to certain poisonous terms (like "root"), or if its "System" field is not set to "UNIX". So check those.
2. Undernet also auto-bans various Names, Emails, and Nicknames (see main ALT-O > Connect category) infamous for being used by botnets (with which Undernet has "occasional" encounters). Check yours and possibly try different settings in all.
3. Finally, a G:line is a global ban. It means your IP address is banned on all Undernet servers. The ban may have been triggered by a false positive against items 1 or 2 above. If so, and even if you fix them, the ban will remain until you either successfully petition them for its removal, or change your IP address.
So, try 1 and 2, and then change your IP address, and then try to connect again. If you're able to get back in, then you'll know it was probably one your old settings that caused the problem, and at that point, if you want, you can begin reverting to your old settings one setting at a time, re-connecting each time, until you find the culprit.
Of course, the cause could also be something entirely unrelated to your settings (how one of your present or past scripts behaved, or even a G:line against your entire netblock caused by the misbehavior of other people).