internet - Can't connect to server, others can

I've set up a Webserver(port 80) and a Minecraft server. Me and my friends play on this server regularly and haven't had any problems connecting before this.

Now, one friend complains about not being able to connect. In order to debug this I made him connect to my webserver(different computer, same IP), but he says he can't.
Pinging doesn't work either.

Other friends however, have no difficulty connecting, so it's not a router issue(right?)

In order to be sure, we both did a system reset on both our routers/modems, still nothing.

I did some research on why this could be, and some say that being on the same subnet results in this kind of behavior. How can i test this?

To sum up:
At first, everybody can connect.
Then out of nowhere, A can't ping B, B can't ping A. But C,D, etc. can ping A and B just fine.

What can I do to fix this?

Edit: We are connected over the internet.

Edit 2: Traceroute
$ traceroute 80.56.241.61
traceroute to 80.56.241.61 (80.56.241.61), 64 hops max, 52 byte packets
1 10.15.141.1 (10.15.141.1) 10.534 ms 9.016 ms 8.072 ms
2 p3129.net.upc.nl (212.142.3.129) 8.804 ms 9.535 ms 9.301 ms
3 nl-ams05a-rd2-ae-30-2368.aorta.net (84.116.244.73) 11.016 ms 11.831 ms 27.244 ms
4 84.116.244.66 (84.116.244.66) 12.686 ms 13.742 ms 11.674 ms
5 84.116.135.30 (84.116.135.30) 11.933 ms 12.267 ms 12.614 ms
6 p60030.net.upc.nl (212.142.60.30) 13.194 ms 12.476 ms 10.680 ms
7 * * *
8 * * *
9 * * *

Edit 3: Called UPC
Fed up, I called UPC again. They told me everything was fine and that it's most likely my server or my router's problem. Thing is, I know for sure it isn't since I reset them both. He did admin however, that it could be a subnet problem but they can't do much about that other than wait until I get a new IP.

Anybody got ANY ideas that don't just mean: wait?

Edit 4: UPC caved, they're investigating it now.

Answer

If your friends are connecting over the public internet then try looking at the output of tracert (windows) or traceroute (unix) too see where the connection is being blocked if your IP address is 1.2.3.4 then

tracert 1.2.3.4

or

traceroute 1.2.3.4

If you are all connected to the same LAN then check that the IP addresses are all in the same block and then netmask for your friends computer is the same as everyone else's.