I am unable to register a database in the ArcGIS Server Manager (ArcGIS Server Manager >Site>Data Stores>Register>Database). When I try to import the .sde file a portal login prompt appears and when I enter my credentials the 'Unable to access the authentication service' error below appears. Note: We use IWA to authenticate so I pretty sure this login prompt shouldn't appear if configured correctly.
Researching the error I found that installing an intermediate certificate to link an intermediate certificate to link the SSL certificate to the trusted root CA may solve the issue. Unfortunately after doing this on the server and portal, the error is still present.
It's weird, but I am able to register a database in ArcMap with no problem. Just in ArcGIS Server Manager does this error exist.
Environment: ArcGIS Enterprise 10.6.1, federated portal using IWA. Error is present in both IE and Chrome
Any help would be appreciated.