[Plugin: WooCommerce] Simultaneous paypal payments fail

Hi folks,
following problem. If a Paypal payment starts while another one isn’t finished yet the first one gets cancelled or more specific, it will set to Order Status refunded and the payment fails.

The system currently runs with WooCommerce 2.3.13 due to compatibility reasons with other plugins. However I manually updated the PayPal Payment Service. The PayPal log file doesn’t give a clue why the payment fails as it only states the start of the first request.

09:50:04 - Generating payment form for order 5176. Notify URL: http://***********************/wc-api/WC_Gateway_Paypal/
09:55:05 - Generating payment form for order 5178. Notify URL: http://***********************/wc-api/WC_Gateway_Paypal/
09:55:52 - Checking IPN response is valid
09:55:54 - IPN Request: Array
(
    [body] => Array
        (
            [cmd] => _notify-validate
            [mc_gross] => 10.70
            [invoice] => SL5178
....

As you can see, there is only one line regarding the order 5176. Do you have a hint why this could happen? Did this problem occur before?

Kind regards and thank you,
Niklas Fechner