Admin locked out. 503 service unavailable

I and my fellow admins are all locked out of our WP admin. As soon as we log in we get a “503 Service Unavailable The server is temporarily busy, try again later!” error. I have tried renaming the plugins folder via FTP but the problem persists.
It happened as soon as we log in to WP and persists from then on. Waiting does not help. I think an over-zealous plugin has banned every person who logs in, even legit admins.
What is the next step, since renaming the plugins folder does nothing?

The page I need help with: [log in to see the link]