Fixing The Error
This might be caused when you have lots of tabs with DelugeRPG open in them. Or if someone tricked you into clicking that link.
If its the first case, go back to the page you were trying an action in. For example, if you were trying to accept a trade, go back to the page that allows you to accept and deny a trade. Refresh the page, just in case, and follow the link again, and it should work fine
Why Does This Happen?
Its to prevent something called a CSRF attack
It is possible for a malicious user to construct a URL (link) that executes an action. On DelugeRPG, one could give you a link that accepts a trade and when you click that, the page would just say something along the lines of 'Trade was successful'. Observant users might be able to detect such scams, but most people simply click every link they see without making sure its safe.
This is where 'Tokens' come in. They let us know that its you who requested the action and that its safe to continue.