Drupal 8.x

I'm building sites in Drupal 8 for managing model railroad conventions and would like to make it possible for the non-technical site maintainers to update and add large amounts of content so that I don't have to do all the work.

SMTP is configured for gmail with port 465 and using SSL for encrypted protocol.

The credentials are all correct but when tried to send a test email, it says 

" SMTP module is active.

• A test e-mail has been sent to xxxx@xxx.com via SMTP. You may want to check the log for any error messages.

      Disabled fields are overridden in site-specific configuration file.

     Unable to send email. Contact the site administrator if the problem persists.

I have a Drupal 8 site built on my local machine with Acquia Dev Desktop. I am installing Drupal on AWS EC2, and am running into the following error during the "Database Configuration" step of installation.

Failed to connect to your database server. The server reports the 
following message: SQLSTATE[HY000] [2002] Connection refused.

I have checked the db name, username, password, and port with my config.inc file, and still can't get it to connect.

This is a screenshot of the error screen:

After installing Drupal 8.4.5 on a live host server, I get this message every time I try to upload something (a theme for example): 

• The file phpkpwzIA could not be saved. An unknown error has occurred.
• File upload error. Could not move uploaded file.

Then in the logs it says: Upload error. Could not move uploaded file nexus-8.x-1.1.tar.gz to destination temporary://nexus-8.x-1.1.tar.gz.

Hello,
I am a beginner with drupal, I make a personal theme, I understood how it works for pages, blocks, views, ...
I would like to format the form 'feedback', and integrate it completely to my theme, but I block.
I found some info on google, but nothing helps, I can not.
I can not fit the fields one by one.

I do not know if I'm clear enough ...

Does anyone have an idea?

Thank you in advance :)

Hello, 

First sorry for my very bad english.

In my module I update a value in database, for exemple :

 $connection = \Drupal::database();
        $connection->update('webform_submission')
          ->fields([
            'current_page' => 'identification_operateur',
          ])->condition('sid', $submission->id())->execute();

The update database is ok, but sometimes I have to do a "drush entup", otherwise my frontend is broken.

Would you have an explanation.

Thanks