Huh .. Just got my new test lab of SP 2010 created and it gives an error saying that “An Unhandled Exception Occurred in Silverlight Application” when I’m trying to create an new item (Library/List or any other new item).
was looking for other’s on web and realized that it’s a common thing. done below steps and restarted IIS, error was gone !
1. Go to Central Admin –> Application Management –> select your web application –> General Settings of particular web Application
2. Direct to Web Page Security Section and switch on the Security Validation if it’s off. sometimes you may turn it off and then turn on if it’s already on then it would work.
that’s all. give a try now with a new browser tab, you wont see the error now. if the problem persist – Restart the IIS of all SP Servers and try again.
Microsoft Cloud Infrastructure Technical Journal 
Sometimes you simply need to add your site on your IE Compatibility View list and it will work.
hii, I had done this but there is no chagne, so can you give any solution for this problem
Hi Manohar,
Thanks for your comment.
Have you restarted your servers and tried as well ? if so try the step Albert has mentioned that you IE Compatibility Settings. and also make sure that you have added your site as trusted. hope you are using IE ?
let me know the result.