Steps to Reproduce

This is reproducible easily. Simply export a page in Confluence to Word. Then try to import the Word document to another page in Confluence.

Cause

If you open the Word document in Microsoft Word, it looks perfectly fine. Editing the same file in notepad++ or any text editors shows HTML markup. When imported back to Confluence, since the file is still essentially a html file with .doc extension, Confluence throws the error message.

Workaround

Export the page to Word, copy all the contents in the Word document (from within Microsoft Word), paste in another blank Word document (This new Word doc will now be a pure Word document, without HTML markup), and import this new Word document to Confluence.