We are not aware of any issues here. When you say the map doesn't load, what do you see on the screen? Do you see the rest of the web page, but the map area is blank? How about the Contents panel? Explorer Online is a Silverlight application, so the way it draws web maps is different.
What's in the web map? Have you loaded shapefiles into it? Can you see a web map with just a basemap in it?
How about other websites? Do these display correctly in the browser? I really can't see anything we're doing that would prevent drawing of web maps.
Thanks,
Mike