The backed up file is already in the profile folder, but it won't recognize it.
As a (torturous) solution, I uninstalled Opera 36, while saving the Bookmark.json file to my desktop. I then reinstalled Opera. Then I tried opening the Boomark file; an Explorer box popped up asking what program I'd like to use to open it. I chose Opera, which opened a tab where the contents of the .json file were dumped in an html table. I scrolled through and copied all the urls, opened them, and then added them to Bookmarks.
Never again, though.
Thanks for your help.