It's complicated (to be honest, more complicated than I understand
). You find explanations on the internet why a shell might issue this message, for example
https://stackoverflow.com/a/11821441/3548109.
In your situation, the problem should be solved with a Bourne Shell instead of Bash. Find out where it resides, perhaps /bin/sh, and use init=/bin/sh.
EDIT: I am currently working on a Xubuntu system, where /bin/sh is a symbolic link to /bin/dash. dash also features job control, but it's size is about a tenth of bash. Perhaps it's better suited than bash. Right now, I can't see how to install the Bourne shell on Ubuntu.