Kenneth:--------------------------------------------------
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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I can't ask my clients to install the Silverlight 5 tools. My clients are organisations and I don't know all the individual users.
Asking them all to downgrade their Silverlight plugin (if at all possible) is simply not an option, nor is asking them all the install any additional tools.
It should work out-of-the-box.
Regards, Marja