I see you're not getting any responses. I'm having a problem that I suspect is related. I'm trying to install FileMaker Server on a new Mac Mini (late 2014) with 3GHz Intel Core i7, 16 GB RAM. I'm running Yosemite 10.10.1. I can install FileMaker Server, but I can't get WebDirect installed.
The reason I think our problems are related is that WebDirect refuses to install, telling me "Test web server (http://127.0.0.1:80/fmi-test/test.xml): Connection refused". Port 80 appears not to be available. Like you, I use Network Utility to do a port scan and find that port 80 is not on the list. You're finding it comes and goes. I'm finding it is never there.
Have you had any success at identifying your issue? If you have, I'd like to know. It might give me a clue where I need to look.
I called support and they advised me to turn off my backup software. We use BRU to backup things to LTO, so I'm testing it right now to see if it works.