Value cannot be null.Parameter name:key error

Hi,
I am migrating a website from SF 3.7 to 5. The site is deployed in azure.
Everything was working fine. But now the site is showing error when I try to edit the page in admin site.
Value cannot be null.
Parameter name: key.
When I check the template for the master page, it shows this error message in the widget area of menu controls.

What is causing this error. Please help. 
Is it related to radcontrol because both these are radcontrols. Or is it related to the BuildAction property. If so, what should be set to this property.

Thanks.

I had the same problem and it had to do with the settings of my Application Pool. Specifically the Identity. It was set to ApplicationPoolIdentity. Once I changed it to NetworkService it worked. I'm not sure if this was related but I also changed Load User Profile to True.