This isn’t a problem with password. It has been corrupted, and this specific corruption is known to be caused by some earlier versions (not sure now, but iirc 5.1.x?)
To recover, you may unzip the DOCX as a normal zip file, edit word/document.xml in a plain text editor to remove double w:cstheme definitions, and replace it in original DOCX. This will completely restore the file without any data loss.
Edit: well, the password protection here may be problem though… if you have password set to edit document, then it may be recovered using steps above; however, if that’s password for opening document, then the ZIP will only contain encrypted package. Then you may use current master (daily from e.g. http://dev-builds.libreoffice.org/daily/master/Win-x86@42/) to open it, because in it there will be an option to continue on error. However, the latter option may loose some information.