I noticed the bridgy webmentions for your twitter posts pointing to the site post about setting up an nginx manual logging webmention endpoint. Good stuff. This is a response so you can see if it works with webmentions from the wild.

curl -A "superkuh flashing a laser pointer by hand down a fiber optic cable" https://vgai.de/webmentions -d "source=http://superkuh.com/welcome-to-webmentions-julian.html" -d "target=https://vgai.de/posts/manual-until-it-hurts/"

I hope it works out for you. A good way to filter out most of the pingback spam is to just drop anything that appears in your nginx POST log more than 3 times. Also, I noticed that your webserver does not support HTTP. That's pretty annoying. HTTP and HTTPS are two great flavors that go great together. Only HTTPS is pretty restricting for a site that involves no money or private information.