I'll also assume you've found numerous other sites with different fixes, none of which worked for you.
I also did the fixes, the rebuilds, all ended up with the 503 error.
I tried remove --auto-remove openvas and reinstalling.
Still error.
How I fixed it:
(this will be a fresh openvas, none of your reports or targets will be saved.)
sudo apt-get remove --autoremove openvas
sudo apt-get purge --autoremove openvas
Go into files and delete anything related to openvas, just drag it into the trash.
Empty the trash
Restart the machine
apt-get install openvas
openvas-setup
Login, create scan jobs and it'll work again.
