Thanks for all of your input. I already tried --salvage and pywallet, both without success.
Let's start from here: how did your backup get corrupted? I often read this, but in general digital files don't just get corrupted out of the blue. Something must have happened to it. "Corrupted" can be a lot of things.
What version of Bitcoin Core were you using when you last used the wallet.dat before you made the backup?
Since the backup is on an external drive, I guess that I maybe once removed the drive to soon (without safe removal). About the version, I am not sure, but a rather old one, as this happened back in 2014 / 2015. Until recently I did not check my backup...