Login screen: [Warning: Property for 'invalid.login' not found]

When going to the login screen on the Go Green test server, this message appeared: [Warning: Property for 'invalid.login' not found].
Please add the property for all languages.

To reproduce (sometimes):

• Log in successfully
• Delete the JSESSIONID cookie, and change the 'hal' cookie to something else
• Enter the cms URL in the address bar again