If it was a Word docx document that you converted to LibreOffice docx, there may be some discrepancies. Try saving the docx (after backing it up of course) as a LibreOffice docx document -- meaning don't try to preserve the Word formatting. Also look at the properties of all images in the document. Make sure the title_text attribute is filled.
I'll look into seeing how that attribute gets filled across the different WP applications and see if there's a way to avoid the error when it's missing.
Looks like it probably has something to do with the changes to the last version of the plugin (v0.2.4). If that's the case, you could always try
v0.2.3 and see if the issue goes away. That would actually help me narrow it down.