Hi,
This error appears due to some mismatched XML tags on your document. It's also possible that this happened due to some Math equations. Just to confirm, does your document contain Math equations? If it does, download the Fix It tool
here, which will help you recover the file, though it may not resolve the root cause. Once you've recovered it, you can copy the contents of the document and paste it to a new document.
After using the Fix It tool, we suggest installing the Office 2013 Service Pack 1 to prevent this error from occurring in the future. Follow the steps
here to download the Service Pack 1.
If the Fix It tool doesn't help you recover the document even after installing the Service Pack 1, we'd like to request a copy. Please upload it to OneDrive and share the link here.
Let us know if you're able to recover the document after following these steps.