In my case, the error was produced by the following,
- I have a self signed ssl certificate in my IIS but I don't use it to browse my ReportManager Site.
- The SQLServer installer detects the certificate and configure the sites to be HTTPS without asking you.
- Open <intalldir>Reporting Services\ReportManager\RSWebApplication.config
- Update the <ReportServerUrl> value (remove the S from https)
- Try to browse. Should work