I have. I have replicated the settings I used on my OpenElec Kodi. It's as though I hit the drill up too many times and can't get back.
I had the same problem using RPI1 but this worked worked for me.
Go to system/appearance/file list and enable "show hidden files and directories" Try backing up again in LibreElec.
Click on System and Kodi Backup then click OK on the screen that pops up. The backup process should then begin immediately placing the backup file in your LibreElec backup folder.