People with one of the original style Google Chromecast devices are pulling their hair out this morning as any attempt to stream offers up an error “Untrusted Device” with no obvious steps to rectify the problem.
The full error suggests your Chromcast “couldn’t be verified. This could be caused by outdated device firmware.”.

While this is likely a mismatch between the security certificate or protocol on your device and on Google’s servers, it may not be an easy fix.
However, advice from IT folk in the online comminities is NOT to do a “Hard Reset”. A full reset like this will wipe the device back to factory settings and it will therefore have no WiFi connection.
It’s possible because of this issue it can’t be setup fresh, and if it has no connectivity it cannot obtain any online fix that Google issues.

Sadly, the best advice is to power down, and wait.
EFTM has reached out to Google for comment, and will update as advice comes to hand about activation.
Update: Tuesday 7.30am:
No statement from Google yet, but they have posted in online forums stating they are aware of the issue:

Trev is a Technology Commentator, Dad, Speaker and Rev Head.
He produces and hosts two popular podcasts, EFTM and Two Blokes Talking Tech. He also appears on over 50 radio stations across Australia weekly, and is the resident Tech Expert on Channel 9’s Today Show each day and appears regularly on A Current Affair.
Father of three, he is often found down in his Man Cave.