As already mentioned here,  you should disable all wpDiscuz addons when you update and test the beta version of wpDiscuz 7. For upcoming stable version of wpDiscuz 7, we'll release new versions of wpDiscuz addons to make those compatible with wpDiscuz 7. All old versions of addons are not compatible with wpDiscuz 7, because of new comment layouts and template functions. In order to avoid errors, we recommend update all addons.

wpDiscuz - Media Uploader 1.4.1

Notice: Undefined variable: uploadMaxFS in /var/www/html/wp-content/plugins/wpdiscuz-media-uploader/options/class.WMUOptions.php on line 37

Fatal error: Uncaught Error: Call to private method WMUOptionsSerialized::getDefaultPhrases() from context 'WMUOptions' in /var/www/html/wp-content/plugins/wpdiscuz-media-uploader/options/class.WMUOptions.php:47 Stack trace: #0 /var/www/html/wp-includes/class-wp-hook.php(288): WMUOptions->saveOptions('') #1 /var/www/html/wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters('', Array) #2 /var/www/html/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #3 /var/www/html/wp-content/plugins/wpdiscuz/options/class.WpdiscuzOptions.php(1283): do_action('wpdiscuz_save_o...') #4 /var/www/html/wp-content/plugins/wpdiscuz/options/class.WpdiscuzOptions.php(1072): WpdiscuzOptions->saveOptions() #5 /var/www/html/wp-includes/class-wp-hook.php(288): WpdiscuzOptions->saveAndResetOptionsAndPhrases('') #6 /var/www/html/wp-includes/class-wp-hook.php(312): WP_Hook->apply_filters(NULL, Array) #7 /var/www/html/wp-includes/plugin.php(478): WP_Hook->do_action(Array) #8 /var/www/html/wp-admin/admin.php(170): do_action('admin_init') #9 {ma in /var/www/html/wp-content/plugins/wpdiscuz-media-uploader/options/class.WMUOptions.php on line 47