Fatal error class subscriber not found

hi,

I get this php error:

Fatal error: Uncaught Error: Class “MailPoet\Models\Subscriber” not found in?/…/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php:67Stack Trace1.MPWA_Frontend_Fields->__construct()/…/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php:392.MPWA_Frontend_Fields::init()/…/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php:2183.mpwa_frontend_fields_init_posts_selection()/home/minispat/public_html/wp-includes/class-wp-hook.php:3244.WP_Hook->apply_filters()/home/minispat/public_html/wp-includes/class-wp-hook.php:3485.WP_Hook->do_action()/home/minispat/public_html/wp-includes/plugin.php:5176.do_action()/home/minispat/public_html/wp-includes/class-wp-query.php:29757.WP_Query->get_posts()/home/minispat/public_html/wp-includes/class-wp-query.php:38528.WP_Query->query()/home/minispat/public_html/wp-includes/class-wp.php:6969.WP->query_posts()/home/minispat/public_html/wp-includes/class-wp.php:81610.WP->main()/home/minispat/public_html/wp-includes/functions.php:133611.wp()/home/minispat/public_html/wp-blog-header.php:1612.require(‘/home/minispat/…’)/home/minispat/public_html/index.php:1713.{main}thrown in?/home/minispat/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php on line 67

what can i do to fix this?
Subscriptions are not working, seems like there is an error in sending mails to the subscriber.

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