Office 365 ProPlus Activation on Mac

We frequently receive call from our endusers who are unable to activate Office 365 ProPlus on Mac.

 

When providing their credentials on the ADFS screen to activate Office 365 ProPlus, the login page refreshes without an error. The user is unable to get passed the login page and can't activate the installation.

 

Resetting the keychain didn't helped.

 

Any ideas on how to troubleshoot? Do you know which browser is used by Office to show the activation login page?

 

Bart