Both .RPP and .RPP-bak are completely corrupt (filled with null bytes). The contents are lost unless you have more backup files.
There are
a few reports of this happening throughout the forum. Finding how to produce such corrupt project files on demand would be awesome.
You can setup automated backup every few minutes in a dedicated folder (up to you to clean it up once in a while as REAPER doesn't purge older backups) with these settings in Preferences > Project: