The security check will not let me sign in

There is a short security check on the sign-in page. Most people never notice it, but it is sometimes blocked. What to do depends on the message shown:

1. "We couldn't load the security check": this usually means a browser extension or an organisation's network is blocking it. Try a private window, or a different network such as a phone connection.
2. "The security check didn't finish in time": try again, and if a checkbox is shown, tick it before requesting the code.
3. "The security check couldn't be completed": refresh the page and try again.