IPN requests not reaching the listener

[Lucas512]

Hello,

i'm not sure if this is the right place for this question.

We have a problem with a WordPress plugin which uses PayPal IPN requests to implement the payment process.

Unfortunately it seems to be that the requests are never reaching our server.  It will try to request 8 times and end up without any status code.
Also in the server log files the requests are not listed. It's an Ubuntu 18.04. with plesk installed. I've tried a lot:

- Disabling Firewall
- Disabling Fail2Ban
- Disabling Nginx redirects

Nothing helps.

On another server, which is from the same hoster, the requests arrive.

Could it be a problem on the PayPal side?

Greetings,

Lucas