Steps to reproduce the issue

After logging into admin a white screen is presented. Refreshing browser page delivers admin control panel.
From control panel I access global config/server and change error reporting from 'None' to any other from drop down and then save. This delivers a white screen. Step back and the page recovers but change is not saved.

Using cPanel file manager I then accessed the configuration.php file and changed line 23 from:

public $error_reporting = 'none';
to
public $error_reporting = 'maximum';

Expected result

Website front end does not show any errors and appears to be operating normally.

Actual result

Admin login and changes to Global System pages generate the error message:

Fatal error: Cannot declare class Joomla\CMS\Mail\Mail, because the name is already in use in /home/widget/public_html/plugins/system/plg_sys_eb_mailer/mailer/Mail_3x.php on line 821

System information (as much as possible)

The website is hosted at Rochen and is using PHP 7.2 and Joomla 3.8.12

Additional comments

Not sure when this error manifested itself - I think it was when I upgraded to 3.9.12

Any suggestion as to how to resolve this or progress my investigation would be much appreciated.

Steve