We were unable to connect to <ArcGIS Server URL> Error: Bad syntax in request. (status code 400)

Error Message

When attempting to make an ArcGIS Server connection in ArcMap, the following error message is returned:

Error:   
We were unable to connect to <ArcGIS Server URL> Error: Bad syntax in request. (status code 400).

Cause

The credentials entered are incorrect.

Solution or Workaround

• For a standalone ArcGIS Server, the credentials are the ArcGIS Server credentials.
• For a federated ArcGIS Server, the credentials are the Portal for ArcGIS credentials.