You can try running the restore via the command line:
Code:
calibredb --restore_database --really-do-it
Alternatively, simply
move the Calibre Library folder, start calibre (it will create a fresh library) and use Add Books, using the
second option: "Add books from directories, including sub-directories (One book per directory, assumes every ebook file is the same book in a different format)"
This will import metadata from the backup metadata.opf files.