Thanks for the quick response. I've just realised that I've posted to the WPF forum instead of the WinForms forum. Are you able to check the problem using the WinForms demo?
In the WinForms demo, I receive the following error...
'For security reasons DTD is prohibited in this XML document. To enable DTD processing set the ProhibitDTD property on XmlReader Settings to false and pass the settings into XmlReader.Create method.'
As the SyntaxEditor.Document.LoadFile() method only takes a file path, I tried changing the DtdSupportEnabled property exposed by the XmlSchemaResolver. This, unfortunately, appears to then expect a DTD for use with validation.