Hi MarjaR,
What I mean is the problem causes by your silverlight 5. I suggest you use silverlight 4. If you must use silverlight 5, please refer to the info below
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It is the microsoft silverlight plug-in bug, silverlight 5 must use with Silverlight 5 Tools for Visual Studio .you can download the tools at http://www.silverlight.net/getting-started, it will fix the problem.
Silverlight will not automatic updates to 5.0,If your client manually update to 5.0, then please let him instal the Silverlight 5 Tools too.
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Regards,
Ken