In a federated environment, ArcGIS Server must authenticate through Portal for ArcGIS to verify if users can access the Server Manager. When changes are made to a federated environment, the following error message is returned at the sign-in page:
Error: Unable to access the authentication service.
The federated environment was recently upgraded and a change was made to the web adaptor in some capacity (e.g. new web server, changed name).
To resolve this issue, perform the following steps:
Warning: The following steps require users to unfederate and refederate the environment. Users are advised to be cautious and not rush through the federation steps to ensure further issues do not arise. Unfederating causes content to become disassociated with the original owners. When refederated, the user that configures the federation becomes the new owner. Content must be re-associated with the correct accounts.