Microsoft has published an announcement in the Message Center on the Office 365 Administration portal to request you to update you federation trust by 23 September 2014. If you don’t do it, your authentication may stop working after this date.

This is because they have updated the certificate associated with the tenant to secure the federation trust.

Message posted

Update your Federation Trust by September 23

On September 23, 2014, you may encounter issues if you haven’t refreshed your federation trust information for your Exchange hybrid deployment. We’ve refreshed your Office 365 Organization ID certificates. Please run the following PowerShell cmdlets to update your federation trust information:
Get-FederationTrust | Set-FederationTrust -RefreshMetadata
Please click Additional Information to learn more.

Additional Information

Published: 9/4/2014 13:56