Add troubleshooting help for "Oops! An Error Occurred"

troubleshooting.rst

@@ -206,3 +206,68 @@ Doctrine semantic configuration under the keys ``driver`` and
 
    Don't forget to keep this value in sync with the version configured in
    ``.symfony/services.yaml``.
+
+Oops! An Error Occurred
+-----------------------
+
+This error message comes from your application and is generated by the default
+Symfony's error template.
+
+The server returned a "500 Internal Server Error"
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+.. image:: _includes/production-error-500.png
+   :alt: A 500 error page in the production mode
+   :align: center
+   :class: with-browser
+
+If your application works locally but you see this message on SymfonyCloud it
+usually means you have a configuraton error or missing a dependency.
+
+To fix this issue you have to inspect application logs, the cause of the error
+is usually specified in the error message:
+
+.. code-block:: terminal
+
+   $ symfony logs all
+   [app] [14-Aug-2020 10:52:27 UTC] [critical] Uncaught PHP Exception Exception: [...]
+   [app]
+   [php.access] 2020-08-14T10:52:27Z GET 500 2.386 ms 2048 kB 419.11% /
+   [access] 78.247.136.119 - - [14/Aug/2020:10:52:27 +0000] "GET / HTTP/1.1" 500 843 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4147.125 Safari/537.36"
+
+If the error happens on a non production environment or on the main environment
+of a non production project you can also enable Symfony's dev/debug mode to
+inspect the cause of the error:
+
+.. code-block:: terminal
+
+   # Enable debug mode
+   $ symfony env:debug
+   # Disable debug mode
+   $ symfony env:debug --off
+
+The server returned a "404 Not Found"
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+New Symfony applications comes without controllers by default. This means
+there's no page to show as a homepage. When running your project locally you
+should have been welcomed with this page:
+
+.. image:: https://symfony.com/uploads/assets/blog/new-symfony-welcome-page.png
+   :alt: The default Symfony welcome page in the development mode
+   :align: center
+   :class: with-browser
+
+But with this page when running on SymfonyCloud:
+
+.. image:: _includes/production-error-404.png
+   :alt: A 404 error page in the production mode
+   :align: center
+   :class: with-browser
+
+This is because SymfonyCloud runs in production mode and as such Symfony shown a
+generic 404 error. To fix this, you will have to `create your first Symfony page
+<https://symfony.com/doc/current/page_creation.html>`_.
+
+If you already created a custom page, check that all your files are commited,
+that you ran ``symfony deploy`` and it succeeded.