Webpart not displaying in the 'Select an Asset -- Webpage dialog' window

Hi all,

We are suddenly experiencing a strange problem with Sharepoint 2010.

A user with the correct permissions tries to insert a link, but when the 'Select an Asset -- Webpage dialog' window opens, the right pane displays this error message. It doesnt matter which library or list we select, which site we are in, or which user attempts to do this.

On checking the correlation ID, the event ID is 89a1, and the message is

Error while executing web part: System.NullReferenceException: Object reference not set to an instance of an object.    
 at System.Xml.Xsl.XslCompiledTransform.Load(MethodInfo executeMethod, Byte[] queryData, Type[] earlyBoundTypes)    
 at Microsoft.Xslt.STransform.GetCompiledTransform()    
 at Microsoft.SharePoint.WebPartPages.BaseXsltListWebPart.LoadXslCompiledTransform(WSSXmlUrlResolver someXmlResolver)    
 at Microsoft.SharePoint.WebPartPages.DataFormWebPart.GetXslCompiledTransform()    
 at Microsoft.SharePoint.WebPartPages.DataFormWebPart.PrepareAndPerformTransform(Boolean bDeferExecuteTransform)