Since Heroku has integrated the piggybank_ssl addon by default, trying:

heroku addons:add piggyback_ssl

or

heroku addons:add ssl:piggyback

results in:

! This add-on is now a standard Heroku platform feature. ! Your apps can already use piggyback SSL.

But, accessing: mywebsite.herokuapp.com results in:

SSL is not enabled mywebsite.herokuapp.com does not have SSL enabled.

My production.rb has:

config.force_ssl = true

What am I missing?

Update

I have read the page SSL on Heroku, but it's obsolete, they suggest to run:

heroku addons:add piggyback_ssl

Which is no longer necessary.

SOLVED !

I don't change anything, and now it's working. It seems that it was a heroku problem.