The domain uptime.rejsemus.dk is being served through Cloudflare CDN. Any Let's Encrypt certificate installed on the origin server will only encrypt traffic between the server and Cloudflare. It is strongly recommended that the SSL option 'Full SSL (strict)' be enabled.
https://support.cloudflare.com/hc/en-us/articles/200170416-What-do-the-SSL-options-mean-
uptime.rejsemus.dk has multiple IP addresses in its DNS records. While they appear to be accessible on the network, we have detected that they produce differing results when sent an ACME HTTP validation request. This may indicate that some of the IP addresses may unintentionally point to different servers, which would cause validation to fail.
[Address=2606:4700:3033::6815:52e3,Address Type=IPv6,Server=cloudflare,HTTP Status=200] vs [Address=104.21.82.227,Address Type=IPv4,Server=cloudflare,HTTP Status=404]
uptime.rejsemus.dk has multiple IP addresses in its DNS records. While they appear to be accessible on the network, we have detected that they produce differing results when sent an ACME HTTP validation request. This may indicate that some of the IP addresses may unintentionally point to different servers, which would cause validation to fail.
[Address=2606:4700:3033::6815:52e3,Address Type=IPv6,Server=cloudflare,HTTP Status=200] vs [Address=172.67.164.153,Address Type=IPv4,Server=cloudflare,HTTP Status=404]