Netbox seems to be happy with this configuration. At least when calling from http, the redirect to https takes place and the page is displayed correctly and with the correct certificate.
PHPipam is a bit stubborn. When calling from http, I don't get a redirect to https, but rather "404 page not found". When calling directly with https, the correct certificate is pulled, but the page is displayed incorrectly and with a "jQuerry error".
I don't want to rule out the possibility that I still have a configuration error somewhere with traefik or even phpipam.