This document will discuss what the "Unknown Login Key" error is, what it looks like, and what you should do when you see it.
This issue is an indicator that the login key for your site has changed and the browser is still trying to access the cached version.
You may see this if/when you:
This issue will typically sort itself out on its own but an easy fix would be to clear your browser cache and refresh the page. Or, you could try restarting your browser and/or your computer.