When runnig the wizard of wpDiscuz, I get a fatal error as soon as I click next. The strange thing is, that when I do the same thing on my staging website, the plugin works.

The Debug log says:
Fatal error: Uncaught TypeError: Cannot access offset of type string on string in /var/www/html/wordpress/wp-content/plugins/wpdiscuz/options/class.WpdiscuzOptions.php:1355 Stack trace: #0 /var/www/html/wordpress/wp-includes/class-wp-hook.php(324): WpdiscuzOptions->saveAndResetOptionsAndPhrases() #1 /var/www/html/wordpress/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #2 /var/www/html/wordpress/wp-includes/plugin.php(517): WP_Hook->do_action() #3 /var/www/html/wordpress/wp-admin/admin.php(176): do_action() #4 {main} thrown in /var/www/html/wordpress/wp-content/plugins/wpdiscuz/options/class.WpdiscuzOptions.php on line 1355