In case of 503 the following does help in some cases:
sudo bash -c 'source /usr/local/etc/library.sh && clear_opcache'
sudo service php8.1-fpm restart
This for a docker installation. You might have to adapt it to non-docker environment.
And in case HPB does not work you can try:
ncc notify_push:self-test
ncc notify_push:setup https://<your_domain>/push