Okay, i found a fix. Sort of... The built-in Paypal Standard's IPN is not working at all.
I had to change the payment gateway to PayPal express but i received this after i clicked on the purchase button:
SetExpressCheckout API call failed.
Detailed Error Message:
Short Error Message:
Error Code:
Error Severity Code:It's caused by the missing php-curl from the server or the curl module is not enabled. I had to contact the server administrator, he enabled it and now the IPN is working with PayPal Express.
For the developers: Please implement a solution to check for curl and when it's not available show a more relevant error message. I found several threads with this issue and there was no solution at all. Most likely all of them are caused by the missing curl.