All the users who are accessing the link via private network is able to stream the file . for few users they are getting the above error while trying to access the link via VPN or MOB or if the user is using a win 7 machine with WMP 12.
I would suggest getting a network trace to find out what is going on between the client and server. Network Monitor 3.4 is available from
http://www.microsoft.com/en-us/download/details.aspx?id=4865. Make sure that the protocol is not being filtered by the VPN (typically WMP will use either port 554 for RTSP or 80
for HTTP). The trace should also show you things like name resolution is happening, TCP port connections, and that you're getting a valid response from WMS.
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