This error appears when you've had a tab of ARC open for 2 hours without any activity, and the error appears because your login session has then expired. Forcing your login session in ARC to expired when you allow a tab of ARC to remain dormant for 2 hours is a security feature.
This error will not always be immediately apparent, but "symptoms" are most commonly seen as issues with the Land Tour 360® editor.
Here is a video demonstration of how a 419 error is typically encountered: How the Error 419 Message Occurs - Video Tutorial