And you haven't got an verification email to attend to at the newuser gmail account?
A new account is not approved until you have responded to the mail, or have approved it manually in the admin page (which seems to be a Catch 22 for you right now).
I don't understand why an IP change would affect the cookie, since that is set with the site name - not IP...
I have a couple of versions running on different machines and cookies seem to be the least problem.
However if there is a change to the site name from eg forum.aaa.com to forum.bbb.com, a new cookie will be created and require a login to create a new cookie.
Could it be as simple as like your web browser or client security software is set to not accept cookies from your forum site?
Just a crazy thought, but still worth mentioning since I've had users with this problem...
Another issue might be an IE "stale cookie" problem, that I have encountered both in my forum and at work...
So - have you tried with different web browser? Like Google Chrome or Mozilla Firefox?
The IE "stale cookie" problem means that you will have to empty the IE cache completely.
Using the IE standard "Internet Options" -> "General" -> Browsing history "😉elete" button will not fix it - so instead of hitting the "😉elete" button, you press the "Settings" button and press "View files". Once there, you press Ctrl-A to mark them all and then press Shift-Delete to remove the files.
I don't know it I'm giving any useful information/advice for this situation, but if you are sure you have the admin password right it could be an IE client side problem - Of course only if you use IE 8-10... First try with Firefox or such, and if it works - delete the IE cached files and try it again with IE.
He who asks a question is a fool for five minutes. He who does not ask a question remains a fool forever. [Old Chinese Proverb]