Webparts Break after editing in SharePoint Designer

Hi, after a site-owner edits a Webpart in SharePoint Designer, the webpart breaks on the site, this happens with even the standard webparts(e.g.when a user edits width,lengh), any ideas how to fix this? See below the error in ULS log.

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)

Thanks

Daren