If I go to your website, I’m getting a “Windows Home Server-Website”, powered by “Microsoft-IIS/6.0”, not some Nextcloud or Rasbian site.
Are you really really REALLY sure your IP address is correct? I’m resolving your FQDN to 87.173.94.238.
Edit:
On port 443, there is a Nextcloud server running, so the IP seems to be correct… But on port 80… No such Nextcloud server, but a Windows server.
Do you have port 80 and port 443 portmapped to different internal IP addresses by any chance? Or are you running two different webservers on the same host? Because your ACME challenge is being put in the Nextcloud webdir, so I’m assuming the IIS webserver doesn’t have any clue about that ACME challenge, but Let’s Encrypt is connecting to the IIS server, as it’s connecting to port 80.