[Defender Pro] Blank Screen in Backend when trying to open Defender Pro

Hi there, when I try to use defender pro in the website a blank screen appears in the backend of the website. I already activated the debug mode and got the following error:

Fatal error: Uncaught Error: Call to undefined function WP_Defender\Behavior\determine_locale() in /html/wordpress-staging/wp-content/plugins/wp-defender/app/behavior/utils.php:1081 Stack trace: #0 /html/wordpress-staging/wp-content/plugins/wp-defender/app/controller/dashboard.php(413): WP_Defender\Behavior\Utils->createTranslationJson('defender-dashbo...') #1 /html/wordpress-staging/wp-includes/class-wp-hook.php(286): WP_Defender\Controller\Dashboard->scripts('') #2 /html/wordpress-staging/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array) #3 /html/wordpress-staging/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /html/wordpress-staging/wp-content/plugins/wp-defender/wp-defender.php(214): do_action('defender_enqueu...') #5 /html/wordpress-staging/wp-includes/class-wp-hook.php(286): WP_Defender->register_styles('toplevel_page_w...') #6 /html/wordpress-staging/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(NULL, Array) #7 /html/wordpress-staging/wp-includes/plugin.php(453): WP_Hook- in /html/wordpress-staging/wp-content/plugins/wp-defender/app/behavior/utils.php on line 1081

Can you please have a look? Thanks a lot in advance.

Daniel Kolks