Okta login UI fails to loan using omniauth-okta

Question

I'm trying to implement oauth with okta on a ruby on rails project. This is what I have at the moment:

# Gemfile
...
gem 'omniauth'
gem 'omniauth-okta'
...

# config/initializers/omniauth.rb

Rails.application.config.middleware.use OmniAuth::Builder do
  provider :okta,
    ENV['OKTA_CLIENT_ID'],
    ENV['OKTA_CLIENT_SECRET'],
    {
      client_options: {
        site:          'https://MY_DOMAIN.oktapreview.com',
        authorize_url: 'https://MY_DOMAIN.oktapreview.com/oauth2/default/v1/authorize',
        token_url:     'https://MY_DOMAIN.oktapreview.com/oauth2/default/v1/token',
        user_info_url: 'https://MY_DOMAIN.oktapreview.com/oauth2/default/v1/userinfo'
      }
    }
end

# config/routes.rb
...
match 'auth/okta/callback', to: 'omniauth#create', via: [:get, :post]
match 'auth/failure', to: redirect('/'), via: [:get, :post]
match 'signout', to: 'omniauth#destroy', as: 'signout', via: [:get, :post]
...

# app/controllers/omniauth_controller.rb

class Omniauth < ActionController::Base
  def callback
    render inline: <<~HTML
      <div>Welcome #{auth['uid']}</div>
    HTML
  end

  def failure
    render inline: <<~HTML
      <div>You reached this due to an error in OmniAuth</div>
      <div>Strategy: #{params['strategy']}</div>
      <div>Message: #{params['message']}</div>
    HTML
  end

  private

  def auth
    @auth ||= request.env['omniauth.auth']
  end
end

<!--views/login/new.html.erb-->

<%= button_to 'Login with okta', '/auth/okta', method: :post %>

At the moment I'm getting this errors when I click the button to sign-in

[uuid] [uuid] [ip] [remote_ip] Started POST "/auth/okta" for 127.0.0.1 at 2022-01-04 13:58:07 -0800
D, [2022-01-04T13:58:07.215161 #22097] DEBUG -- omniauth: (okta) Request phase initiated.
W, [2022-01-04T13:58:07.215791 #22097]  WARN -- omniauth: Attack prevented by OmniAuth::AuthenticityTokenProtection
E, [2022-01-04T13:58:07.216301 #22097] ERROR -- omniauth: (okta) Authentication failure! authenticity_error: OmniAuth::AuthenticityError, Forbidden
E, [2022-01-04T13:58:07.216640 #22097] ERROR -- omniauth: (okta) Authentication failure! Forbidden: OmniAuth::AuthenticityError, Forbidden
[uuid] [uuid] [ip] [remote_ip] Forbidden excluded from capture: DSN not set
[uuid] [uuid] [ip] [remote_ip]
OmniAuth::AuthenticityError (Forbidden):
  omniauth (2.0.4) lib/omniauth/authenticity_token_protection.rb:27:in `deny'
  rack-protection (1.5.5) lib/rack/protection/base.rb:53:in `react'
  omniauth (2.0.4) lib/omniauth/authenticity_token_protection.rb:18:in `call!'
  omniauth (2.0.4) lib/omniauth/authenticity_token_protection.rb:11:in `call'
  omniauth (2.0.4) lib/omniauth/strategy.rb:240:in `request_call'
  omniauth (2.0.4) lib/omniauth/strategy.rb:193:in `call!'
  omniauth (2.0.4) lib/omniauth/strategy.rb:169:in `call'
  omniauth (2.0.4) lib/omniauth/builder.rb:45:in `call'
  ...

# lib/omniauth/authenticity_token_protection.rb

def deny(_env)
  OmniAuth.logger.send(:warn, "Attack prevented by #{self.class}")
  raise AuthenticityError.new(options[:message]) # <<--- fails here
end

I'm currently scratching my head since I'm unsure on what's the problem: a missing conf key, a missing conf on okta, a typo on the okta endpoints, ...

Note: the project uses rails (4.2.11.3), omniauth (2.0.4), omniauth-oauth2 (1.7.2), omniauth-okta (1.0.0)