Error=Object reference not set to an instance of an object. at System.Xml.XmlLoader.Load(XmlDocument doc

I have a SharePoint 2010 site. When I run Approval Workflows I get an error message. The log file says,

 Error=Object reference not set to an instance of an object.   at System.Xml.XmlLoader.Load(XmlDocument doc

I have activated all relevant features.What is missing. Any clues.

thanks a lot

Mayank