POST …/admin-ajax.php 500 (Internal Server Error)

Hi
I had the same problem. I solved it with these 2 changes:
1. On index.php line 53, change

include_once SEPA_PAYMENT_GATEWAY_PATH.'/vendor/autoload.php';

For this:

include_once SEPA_PAYMENT_GATEWAY_PATH.'vendor/autoload.php';

2. On implement.php line 108, change

$url_download = SEPA_PAYMENT_GATEWAY_LOG_URL.$filename;

for this:

$url_download = SEPA_PAYMENT_GATEWAY_LOG_URL . '/' . $filename;

I hope it works for you.

Regards