I did a test, created a new user "Administrator2" and added it to the local administrator group. When I use this account in configuration manager it works fine.
Can you open the trace log to see more detailed error messages? Its path is typically "C:\Program Files\Microsoft SQL Server\MSRS13.MSSQLSERVER\Reporting Services\LogFiles" or "C:\Program Files\Microsoft SQL Server Reporting Services\SSRS\LogFiles".
From what I've searched, 401 errors generally occur when the user is not authorized by the report server or authenticated with Kerberos but not registered with the SPN. So make sure your account is connected to the report server or Kerberos is properly configured.