Hi Erich,
If you set EOS_MGM_HTTP_PORT=8000 then everything should work - you will have the old libmicrohttpd running on 8000 (which is unused in your case) and XrdHttp running on 8443. If you set this value to 0 then the internal HttpServer is not crated at all and the XrdHttp does need it to work properly. All this will be soon cleaned up in the EOS 5 version.
Cheers,
Elvin