PayPal Listener doesn't work (Uncaught Error)

Dear support

Product and Platform:
Chronoforms 5.0.14 / ubuntu 16.04.1 / PHP 7.0.18-0ubuntu0.16.04.1

I have a problem with the PayPal listener module.
It seems that the module is broken.
If I access the IPN address (normal or IPN Simulator), I receive the following error message in the Apache error logfile:

PHP Fatal error: Uncaught Error: Call to undefined function GCore\\Admin\\Extensions\\Chronoforms\\Actions\\PaypalListener\\curl_init() in /var/www/administrator/components/com_chronoforms5/chronoforms/actions/paypal_listener/paypal_listener.php:51\nStack trace:\n#0 /var/www/components/com_chronoforms5/chronoforms/libs/form.php(106): GCore\\Admin\\Extensions\\Chronoforms\\Actions\\PaypalListener\\PaypalListener->execute(Object(GCore\\Extensions\\Chronoforms\\Libs\\Form), '25')\n#1 /var/www/components/com_chronoforms5/chronoforms/libs/form.php(84): GCore\\Extensions\\Chronoforms\\Libs\\Form->runAction('paypal_listener', '25', Array, Array)\n#2 /var/www/components/com_chronoforms5/chronoforms/chronoforms.php(44): GCore\\Extensions\\Chronoforms\\Libs\\Form->process(Array)\n#3 /var/www/libraries/cegcore/libs/app_j.php(249): GCore\\Extensions\\Chronoforms\\Chronoforms->index()\n#4 /var/www/libraries/cegcore/libs/app_j.php(444): GCore\\Libs\\AppJ->dispatch(true)\n#5 /var/www/be- in /var/www/administrator/components/com_chronoforms5/chronoforms/actions/paypal_listener/paypal_listener.php on line 51

I added a mail module before the IPN listener module to verify the date from PayPal:
$_POST: Array ( [transaction_subject] => [payment_date] => 00:49:37 Jul 10, 2017 PDT [txn_type] => cart [last_name] => Buyer [residence_country] => CH [payment_gross] => [mc_currency] => CHF [business] => info-facilitator@test.net [payment_type] => instant [protection_eligibility] => Ineligible [num_cart_items] => 1 [verify_sign] => AIkKNboJiyuxWLOHUlzTd3lpqCSxAAYNTjCced2dhSe7KYoqtJxqpaxV [payer_status] => verified [test_ipn] => 1 [payer_email] => info-buyer@test.net [txn_id] => 0RM264386F252692F [receiver_email] => info-facilitator@test.net [first_name] => Test [payer_id] => 4865EIZJ262ZA [receiver_id] => T9AWS8MPKVZ56 [payment_status] => Completed [payment_fee] => [mc_fee] => 1.26 [mc_gross] => 21.00 [custom] => 90 [mc_gross_1] => 21.00 [item_name1] => Test [charset] => windows-1252 [notify_version] => 3.8 [item_number1] => [quantity1] => 1 [ipn_track_id] => def2a4f597dce [Itemid] => 250 )

I have no idea, what's wrong.
Some moments ago, my configuration worked very fine.

Many thanks for your support.

Best regards,
readych

You're absolutely right!

This solves my problem.

Many thanks.
Readych