Try setting EnableAjax="false" on the RadAjaxManager for debugging. This should allow you to see where the underlying error is happening. Oftentimes the "Value cannot be null: Parameter name ____" is caused by a LINQ-to-SQL or LINQ-to-Entities call that has some bad data. Specifically, it can happen when LINQ expects a field to be non-null, but the data coming from the database has a null value. Let me know if that helps.