After a bit of research, I found out that the problem might be generated from entity.inc inside the includes folder.
Therefore, I tried to put back the previous entity.inc (from the 7.65 installation); subsequently, I run the update.php script, and after another refresh, the website is finally accessible.
I done few more tests to check if there are ...
In my case I was getting a Drupal error
Error: Call to undefined function Drush\UpdateService\simplexml_load_file() in Drush\UpdateService\Project::getInstance() (line 74 of phar:///usr/local/bin/drush/vendor/composer/../../lib/
which indicated that the php-xml extension was not installed.
sudo aptitude install php7.2-xml
fixed the ...
You are using the tilde here:
From experience I know it is better to pinpoint drupal/core to a certain version. FE "drupal/core": "8.6.17",
This leaves composer with no other choice than going for that version of core.
In case you are using the webflo template you have to run another command:
composer update drupal/core ...