I am not quite sure what the problem could be here. For it looks some issue related to the OS and Silverlight version you use. Can you try the following
1. Revert back to the previous version of Silverlight and see whether the problem will appear.
2. Try to uninstall and then install the Silverlight and see whether the problem will appear.
3. If 1 and 2 do not give you positive result, can you move the project to your local server that uses Windows 7 and try to replicate the issue?
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