I'm trying to capture the traffic happening when a domain-joined client uses Outlook Web App to log on to his Office 365 account. The whole setup is rather standard, with an on-premises Active Directory Federation Services instance that issues a security token to the user, which in turn hands this over to the Office 365 service. Without Fiddler everything runs smooth and the client logs on just fine. However if Fiddler is used, right when the 302 redirect happens from the O365 page towards the ADFS one on-premises, I'm prompted with a credential dialog, which just won't go away, regardless the values used are ok.
Could I have a pointer to what should I check in order to understand at least the cause of the credential prompt ?