on win 7 C# WPF application OSM error: unable to contact the server to download map data

I am trying to access OSM data from C# WPF application in VS2013 on win7.

The code is at: http://www.codeproject.com/KB/WPF/MapControl/MapControl.zip

When I opened it (from my laptop ) and zoomed in, I got error:

unable to contact the server to download map data.

when I visited

 http://tile.openstreetmap.org/3/3/3.png

from the WPF applciation. If I opened the WPF application in a remote server (with win server 2008 and win 2012), no problems.

But, about two months ago, I could open the same application on my laptop without problems.

I can access OSM a and zoom in deeper by visiting OSM from web browser

 http://tile.openstreetmap.org/3/3/3.png

It is a WPF application and I cannot see the URL when I zoomed in. I felt that when I zoomed in above level 4, i got the error: unable to contact the server to download map data.

I also tested it on my desktop with win7, when I opened the WPF applciation, I got the same error.

Any suggestions ? thanks