The Office Open XML file cannot be opened because there are problems with the contents. Details: The name in the end tag of the element must match the element type in the start tag. (Sibling detail messages: "XML parsing error", "An XML declaration is only allowed at the start of the document" — Location: Part: /word/document.xml, Line: X, Column: Y)

What's actually wrong

These detail messages all mean the same thing: one XML tag inside your document is malformed, so Word's strict parser refuses the whole file. The end-tag mismatch is a documented Word bug — it happens when an equation (oMath) shares a paragraph with an anchored graphic or text box, and Word writes the closing tags in the wrong order when saving. The good news: the file is complete and the damage is usually a single spot, so recovery is often 100%, formatting included.

Free fixes — try these first

1. Open and Repair: File → Open → Browse → select the file → arrow next to Open → Open and Repair.
2. Open in LibreOffice Writer and re-save as .docx: LibreOffice tolerates the tag mismatch, and its re-save rewrites the XML correctly. This is the highest-success free fix reported in forum threads.
3. Round-trip through Google Docs: upload, open in Google Docs, File → Download → .docx.
4. Fix the tag by hand: copy the file, rename to .zip, extract, open word/document.xml in Notepad++ → Plugins → XML Tools → Pretty print, then jump to the line and column from the error dialog. Look for a mismatched pair — usually around (equation) blocks — correct the end tag name (or delete the broken element pair), re-zip, rename back to .docx.
5. Prevent it recurring: if the file came out of Word itself, avoid anchoring text boxes or images to the same paragraph as an equation; if it was generated by other software (Trados, PHPWord, an export tool), regenerate after removing equations/text boxes, since the generator is writing invalid XML.

If the free fixes fail

The mismatch is typically buried in one enormous line of XML where a hand edit can silently corrupt more of the file — our analyzer locates and repairs mismatched and misplaced tags across every part of the archive and shows a percent-recovered preview, so you know before doing anything whether the equation-heavy sections survived.