Hi Ludek,
I tested with a machine using the latest 1.29.3 (Build 99) on our corporate network. I'm still getting the same error message.
Here is info from the error log:
XMLHttpRequest cannot load http://www.arcgis.com/sharing/rest/portals/self. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://127.0.0.1:65527' is therefore not allowed access. The response had HTTP status code 407.", source: http://127.0.0.1:65527/h5v2/index.html?securityToken=XXX&locale=en
I suspect corporate IT is blocking the communication.
However, I'd like to get more details before approaching our network administrators.
Could it be the ESRI port 6080 (see ESRI ArcGIS ref) is blocked?
Andrew