Hi! upgraded from 3.9 to 4.0 and i got 500 error after admin login
[27-Sep-2020 12:13:00 Europe/Bucharest] PHP Warning:  require(/home/xxxxxx/xxxxxx/oc-admin/themes//main/index.php): failed to open stream: No such file or directory in /home/xxxxx/xxxxxxx/oc-includes/osclass/helpers/hDefines.php on line 185
[27-Sep-2020 12:13:00 Europe/Bucharest] PHP Fatal error:  require(): Failed opening required '/home/onejobro/oneauto.ro/oc-admin/themes//main/index.php' (include_path='.:/opt/alt/php74/usr/share/pear') in /home/xxxxxxx/xxxxxxxx/oc-includes/osclass/helpers/hDefines.php on line 185



This is in hDefines.php on line 185


function osc_current_admin_theme_path($file = '') {
        require AdminThemes::newInstance()->getCurrentThemePath() . $file;
    }

Do you have main/index.php file?

On that path you see in error

@Nagy
Ok I see now problem, you miss "omega" word in URL, there are 2 possible problems:
1) oc-admin/themes/omega folder does not exists. If not, copy it from downloaded zip
2) In oc_t_preference table, make sure that row with s_name = "admin_theme" has s_value set to: omega