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