https://thp.reisen/.well-known/matrix/server points to matrix.thp.reisen:8448 and 8448 is not proxying matrix API, but matrix.thp.reisen:443 does. You probably want to correct your well-known endpoint so it would to point to matrix.thp.reisen:443, or it would be difficult for other servers to successfully communicate with yours. https://federationtester.matrix.org/#thp.reisen can be used to test correctness of configuration.
