If you are using Office 365 and have implemented ADFS to manage the authentication to Office 365 services, this post is for you 
You may get the following error code – 8004789A – when logging on to Office 365 with web browser (Exchange Online OWA or SharePoint Online), while Outlook or any ActiveSync device is working fine
The solution is simple:
check if the Issuer (URI) URL is set to HTTPS; if the answer is yes, you have to change the configuration to use HTTP only
run the following command from a PowerShell window on the ADFS server
Add-PSSnapin Microsoft.Adfs.PowerShell
Set-ADFSProperties -Identifier "http://<your ADFS URL>/adfs/services/trust"
Restart-Service adfssrv
Also on the TMG side, check if the following settings are correctly set:
- Apply link translation to this rule – available on the rule property below the Link Translation tab – is disabled
- Verify normalization – available on the HTTP policy property – is disabled
