Error with domain validation

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Hi all, I’m getting the following error when I’m trying to pass the domain validation. Does someone know how to solve this?

The domain is not reachable on Port 443 from within this container. Have you opened port 443/tcp in your router/firewall? If yes is the problem most likely that the router or firewall forbids local access to your domain. You can work around that by setting up a local DNS-server.

Port 443 is open. Nextcloud is running on a host on a vlan with no access to other vlans.