Local revision could not be obtained, => segmentation fault :-/

Is this a new (recent) installation or an old installation?

Apparently your repositories are broken, causing git errors:

  • git status is failing in /etc/.pihole
  • git rev-parse --abbrev-ref HEAD is failing in /var/www/html/admin

Try the steps suggested here to fix the repositories and try pihole updatechecker again.

If still fails, you can try to repair your installation with pihole -r and select repair, but this won't work if your SD card is failing.