It's your system. Find and change that. There are different places possible - firewall, htaccess, router, application.
That's unrelevant, that's your browser cache. See https://check-your-website.server-daten.de/?q=files.prsnl-server.com%2F.well-known%2Facme-challenge%2F1234 - there is no redirect to https.
Read your own error message.