Make sure you don’t have any external firewalls blocking inbound HTTP traffic to your server. Outbound may be fine, but inbound HTTP on port 80 does not appear to be working for you.
That’s evident from the message shown in your logs “…but HTTP validation failed”
From outside of your server (e.g. your workstation, something outside the network where your mailcow server resides), verify connectivity to port 80:
nc -zv <server-ip> 80
If it works you should see a message similar to the following:
Connection to <ip> port 80 [tcp/http] succeeded!