General help

The second screenshot shows a call to /admin/settings/privacy.php. That file isn't part of Moodle 3.8 and shouldn't be there (it was removed in an update to Moodle 3.4).

The problem with third party installers like Installatron is that they're not from Moodle so we don't know exactly what steps they perform which makes it difficult to provide support. Presumably Installatron has left some files behind.

I would start by removing the above file (e.g. rename it to privacy.php.old) and see if the problem still occurs. But you don't know if there are other files left behind. Rogue files in some locations won't cause problems but in locations such as /admin/settings they will. The best way forward would be to move from the current source code folder to one obtained from Moodle.org along with the third party plugins and the original config.php. This task would need planning and I realise that, depending on the skills available, this may not be an easy job.

As I understand your provider has upgraded your Moodle from 3.4 to 3.8 and since then you have this problem. Then they must have make a mistake. Of course we don't know how they upgrade, but you can ask them whether they are aware of the different server requirments:
- https://docs.moodle.org/dev/Moodle_3.4_release_notes#Server_requirements
- https://docs.moodle.org/dev/Moodle_3.8_release_notes#Server_requirements

For example release 3.4 runs with PHP 7.0 but release 3.8 wouldn't. Similar version incompatibilities with MySQL. I maintain a compatibility list here: http://www.syndrega.ch/blog/#php-and-dbms-compatibility-of-major-moodle-releases.