Is it possible to detect what is blocking port 80

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I get the same result on my machine:

$ curl http://94.15.100.134/
curl: (7) Failed to connect to 94.15.100.134 port 80: Connection refused
$ curl http://127.0.0.1
curl: (7) Failed to connect to 127.0.0.1 port 80: Connection refused

The latter is because I don't have an HTTP server running on my machine; unless you have an HTTP server running on your machine, you should expect that telnet localhost 80 will return "Connection refused".

Are you certain that 94.15.100.134 has an HTTP server listening on port 80? If I run Wireshark while running curl http://94.15.100.134, it shows the initial SYN trying to connect to 94.15.100.134, followed by a RST coming back from 94.15.100.134, which is not a sign that 94.15.100.134 has an HTTP server. The same thing happens if I capture on lo0 and try to fetch from 127.0.0.1, so the RST might be a sign that nobody's listening on that port.

Wireshark will not help here, as a gentle reminder Wireshark is a packet analyser, e.g. a microscope and not a network swiss army knife.

More appropriate tools for the task would be netstat or lsof on the server host to determine if the ports are open, and if so, by which process.

Your use of a private local IP address (192.168.0.15) is confusing me. This address is not routable over the internet and cannot be connected to from a host outside of your local LAN. You can use port forwarding on your firewall so that external connections to the external internet IP of the firewall are forwarded on to the local IP.

You could help by adding some basic info to your question such as where and what the server is and where and what the client is and any known intervening devices such as firewalls.