Webhooks with HTTPS/SSL not working

[#1]

I have a problem sending Webhooks to a HTTPS endpoint.
I've read about a similar problem here
I tried two differen servers, one with a Letsencrypt-Certificate (the devel-machine) and one with DigiCert-certificate (the live-machine). Both don't work. If I test a Webhook with the devel-machine without HTTPS it works, the live machine does not allow non-HTTPS.
If I simulate the call with Postman it works.

Update
When I switch logging to debug-level on the server, I get the following Apache-SSL info:
AH01998: Connection closed to child 4 with abortive shutdown

[Derek_Gervais]

Hi @Heartwork,

Can you message me directly with the URL you're seeing issues with? Including the Hub ID of your developer portal would be helpful, as well.