I would like to know also. I just started getting this error. I removed everything I had changed, but still got it. The only other thing I did was install ArcGIS API 3.0 for Silverlight. I don't know what that would have done to cause this error.
I am running 3.0 as well but I set the references to pull from 2.4 version and it still doesn't work...Do I need to do something else like add in the files from this download code to the viewer template? I am new to working with the viewer template. Also are you running Silverlight 5.0? My viewer template doesn't seem happy with SL 5.0 or API 3.0.