However, clients keep encountering forms-based authentication (FBA) when they try to log on to OWA or ECP.
Note Clients can log on successfully after they provide the correct credentials.
To resolve this issue, install the following cumulative update:
2936880 Cumulative Update 5 for Exchange Server 2013
To work around this issue, make sure that you specify -FormsAuthentication $false when you disable FBA and enable other authentication types in the OWA or ECP virtual directories. To do this, follow these steps:
Enable forms-based authentication.
For OWA, run the following command:
Set-OwaVirtualDirectory -Identity "CAS1\owa (Default Web Site)" -FormsAuthentication $True
For ECP, run the following command:
Set-ECPVirtualDirectory -Identity "CAS1\ECP (Default Web Site)" -FormsAuthentication $True
Enable the authentication type that you want, and specify -FormsAuthentication $false.