Ubuntu: Déjà Dup Backup Failed to Execute Child Process
It's just the way of Linux, especially on legacy hardware: every now and then you screw things up beyond repair--like when you change graphics drivers and can no longer access a login screen. I recently got myself in a pinch and, after hitting my head against a wall for several hours, decided that I'd look at reinstalling Ubuntu and restoring from a Duplicity/Deja Dup backup. Unfortunately, my first attempts to restore the backup presented me with an error that seems to be somewhat common. The fix, as often seems to be the case, was a simple permissions change.
Background
I should know better than to mess with drivers on my old Mac Pro -- the hardware is finicky at best. Out of curiosity, though, I decided the other day to switch to an Nvidia driver. Big mistake!--I soon found myself unable to get to the login screen, and attempts to even get into the command line weren't particularly fruitful (an issue, I suspect, with the boot disk being encrypted).