Der im Endtag des Elements enthaltene Name muss mit dem Elementtyp im Starttag übereinstimmen? (The name contained in the end tag of the element must match the element type in the start tag?)

I received this message when I tried to open a saved target file in Microsoft Word. As far as I can see the tags at the beginning and end of the document are identical? I cannot now send the translation to my client.