Silverlight Error 2103
In my case this wasn't the issue as I had not changed the namespace and additional point was that if I used the auto generated test page, the application was working fine, but there was this error when used from my custom asp.net page.
I spent many hours trying to fix this. Since it was working for a test page but not my custom one, I got suspicious of if Silverlight had any issues in being used inside of a 'fieldset' tag in ASP.NET page or was it due to my using it inside of a 'MultiView' control? These didn't make sense however.
After a couple of hours of wasted time, it finally struck me. The test page like all typical Silverlight web applications was in the root folder. However the page from where I was invoking my Silverlight XAP file was inside of a sub-folder. The main issue was that the XAP file wasn't found (as relative path is used) and hence the error. If only Microsoft will fix it and raise more appropriate error messages. Something like "XAP file not found", will be so much more easier to understand in this case.
<param name="source" value="/ClientBin/FileUpload.xap" />
I had to only add the extra "/" just before ClientBin in the path above and things started to work. I think this is a change that can easily be made as part of the default template implementation for all Silverlight projects as ClientBin will almost always be a root level folder.