Critical Webhook Error

I installed WMPL and made my page multilingual. Since then, I get the following emails after a payment was made (also the payment seems to be successfully).

“Bei einer Webhook Anfrage ist ein Fehler aufgetreten. Hier finden Sie die Details: https://<mypage&gt;.payrexx.com/cadmin/index.php?cmd=checkout&act=api&tpl=webhookLogs&id=…..”

With debug in wordpress on, I get the following error on the api page

https://www.nussfee.ch/?wc-api=wc_payrexx_gateway

DEBUG output:
”

Notice: Undefined index: transaction in /var/www/nussfee.ch/wp-content/plugins/woo-payrexx-gateway/woo-payrexx-gateway.php on line 134

Notice: Trying to access array offset on value of type null in /var/www/nussfee.ch/wp-content/plugins/woo-payrexx-gateway/woo-payrexx-gateway.php on line 134

Notice: Trying to access array offset on value of type null in /var/www/nussfee.ch/wp-content/plugins/woo-payrexx-gateway/woo-payrexx-gateway.php on line 134

Fatal error: Uncaught Exception: Missing transaction status in /var/www/nussfee.ch/wp-content/plugins/woo-payrexx-gateway/woo-payrexx-gateway.php:144 Stack trace: #0 /var/www/nussfee.ch/wp-includes/class-wp-hook.php(292): WC_Payrexx_Gateway->check_webhook_response('') #1 /var/www/nussfee.ch/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters('', Array) #2 /var/www/nussfee.ch/wp-includes/plugin.php(484): WP_Hook->do_action(Array) #3 /var/www/nussfee.ch/wp-content/plugins/woocommerce/includes/class-wc-api.php(161): do_action('woocommerce_api...') #4 /var/www/nussfee.ch/wp-includes/class-wp-hook.php(292): WC_API->handle_api_requests(Object(WP)) #5 /var/www/nussfee.ch/wp-includes/class-wp-hook.php(316): WP_Hook->apply_filters(NULL, Array) #6 /var/www/nussfee.ch/wp-includes/plugin.php(551): WP_Hook->do_action(Array) #7 /var/www/nussfee.ch/wp-includes/class-wp.php(388): do_action_ref_array('parse_request', Array) #8 /var/www/nussfee.ch/wp-includes/class-wp.php(750): WP->parse_request('') #9 /var/www/nussfee.ch/wp-include in /var/www/nussfee.ch/wp-content/plugins/woo-payrexx-gateway/woo-payrexx-gateway.php on line 144
Es gab einen kritischen Fehler auf deiner Website.

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