To bypass some authentication issues i used https git repo addresses as the default push destination.
It seems like this method is sometimes not supported by some Heroku application (very random and unexpected behavior) and triggers the error :
fatal: unable to access 'https://git.heroku.com/<app_name>.git/': The requested URL returned error: 504
[x] provide an option to use HTTPS address or just remote name (to use SSH).
Addendum: my assumption was incorrect about choosing HTTPS or SSH as Heroku only supports HTTPS transport.
Nevertheless providing only the remote name in git push option seems to avoid the 504 error.
To bypass some authentication issues i used
https
git repo addresses as the default push destination. It seems like this method is sometimes not supported by some Heroku application (very random and unexpected behavior) and triggers the error :