I receive a docx document from a word user, edit it, store it, exit and restart libreoffice, and try to open the edited document. I get error message "SAXParseException: '[word/document.xml line 2]: Attribute w:cstheme redefined ', Stream 'word/document.xml', Line 2, Column 8214(row,col)." When checking the mentioned file, there are several occurrences of w:cstheme="minorHAnsi" occurring twice in the same element. When removing this (with sed) the file opens perfectly. I cannot currently share the file, but will add it when possible.
This bug has been fixed, hopefully completely, please try with a current version (6.0.4 at the moment). Note that an incorrectly saved file is still unopenable without manually correcting the XML inside the DOCX, but the duplicate attributes shouldn't be introduced anymore. *** This bug has been marked as a duplicate of bug 113790 ***