I've mentioned the cause of this once or twice before. To spare you the long explanation, all it's really caused by is you trying to log in from "www.roleplayerguild.com" and not "roleplayerguild.com", since some browsers see the two as two separate websites and save different sets of cookies for them.
Before the site upgrade, it was possible to stay logged in on "www.roleplayerguild.com" and go to "roleplayerguild.com" and be logged out- as your browser had only saved cookies for "www.roleplayerguild.com" whereas you were believed to not be logged in on "roleplayerguild.com". From there, it was actually possible to log in as a different account on "roleplayerguild.com", and remain logged in from two different accounts at the same time, actively using both, one one browser.
vBulletin apparently "fixed" this glitch when the forum upgraded (and I have "fixed" in quotes because it seems to have caused more problems than it solved) by simply not allowing anyone to log in from "www.roleplayerguild.com". As you probably haven't noticed, you've been trying to log in from "www.roleplayerguild.com", to which the site stopped you, and, when you logged in a second time, you were actually taken to "roleplayerguild.com". It's just vBulletin's way of stopping people from getting online from various alts at once.
So, there's a super easy fix here! Stop using "www.roleplayerguild.com". Just make sure you're already on "roleplayerguild.com" when you try to log in.