In case of you missed it, Microsoft has published an announcement in the Message Center on the Office 365 Administration portal to request you to update you federation trust by September 23rd. 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.

To update your trust, just logon on your Exchange server (if you run a farm of servers) and run the command Get-FederationTrust | Set-FederationTrust –RefreshMetadata using an Exchange PowerShell command prompt, as usual executed using the run as administrator.

 

Here is the 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