Well, had another problem all of a sudden with SP 2010.
I wasn’t able to create any list or library because this error kept popping up:
An unhandled exception occurred in the Silverlight application.
Thinking back over what I’d changed… I had recently disabled Security Validation on a web application because I was having issues generating/adding open XML (Office) documents. Surely that couldn’t be it…
Sure enough, it was exactly that.
The Fix
Go to Central Admin → Manage Web Applications → General Settings of the affected web application → re-enable Security Validation.
The error was gone immediately.
Why It Happens
The short explanation (from this TechNet thread):
The Silverlight Application is unable to connect to the WCF endpoint configured by the product for enabling Client Object Model, if Security Validation is set to Off.
It’s a stupid error to have, but an error nonetheless. Hope this saves someone some time! 😊