Error 53 - File not found error when adding a document template category

The following error occurs when:

• The file-system header folder does not exist (in this case "\\MT\Corporate\General\") or
• The "default.docx" file does not exist in the parent folder (in this case "\\MT\Corporate\General\")

Check the windows file-system and make sure the category exists and the "Default.doc" or "Default.dotx" exists: