SSRS 2016 – Client found response content type of ”, but expected ‘text/xml’

I’ve got SQL Server 2017 Developer installed on my workstation, and just installed SSRS 2017. I am able to navigate in the SSRS portal, and upload files of dummy data, and create KPIs. However, when click “+New”, then “Dataset” or “Data Source”, I get the message “Something went wrong. Please try again later.” The only clue in a log file is in my RSPortal_yyyy_mm_dd……log file which shows the following:

OData exception occurred: Microsoft.SqlServer.ReportingServices2010.RSConnection2010+MissingEndpointException: The attempt to connect to the report server failed. Check your connection information and that the report server is a compatible version. —> System.InvalidOperationException: Client found response content type of ”, but expected ‘text/xml’

I get the same error if trying to upload an .rdl file.

I did a “next/next/next” install of SSRS, keeping the defaults. The only unusual thing is that I am logged in with a machine account, not a user account. This machine account does show in the top right corner of the SSRS web portal, so that is passing through at least.

What could be happening here?