port 80 showing filtered - router has it open.

[Linux]

Hello, We have a problem. We have a router which has, amongst others, ports 80 and 443 open. Both are pointing to one machine running apache2. However, https on 443 works, but http on 80 doesn't. nmap shows port 80 as filtered (why?), and port 443 as open. I have stopped the firewall...