Failed to Verify Username

Today, I was on my server, updating and finishing some permissions and plugin modifications. When I finished, I logged off. Normal, right? Yeah. Well, about half an hour later, I tried to login... yet, I got that message:
"failed to verify username!".

I've checked YouTube, Google, forums, and whatever else, but no tutorial/fix has been able to fix this for me! A friend of mine is also having the same issue, he can't login. Anyone know what could be the issue?

Thanks!