I can't solve this issue. Here's in detail what I've tried.
First of all I've uninstalled the WebServer/IIS on my WHS2011. Not happy about that, but for running Plex, ok.
Found only one process with Process Explorer using port 80 (no Skype or anything other)
the System process itself with the PID 4 (I can't end this. If so the server is dead an need a hard reboot as expected)
Checked with netstat -anbo which program or service blocks port 80.
Found two. The first is the mentioned system process. The second one is the SMSvcHost.exe.
Set up in the firewall an inbound and an outbound rule for python.exe for all protocols and all ports. (Allow all in every case)
Before killing the SMSvcHost.ext I've killed the Windows Process Activation service. To end this the following two processes had to be killed additionally.
Net.Tcp Listener
Net.Pipe Listener

No change, Port 80 still blocked.
Then I tried to kill the SMSvcHost.exe. Killed the process with the Process Explorer. After that there was nothing else besides the following in the result of netstat.
[System]
TCP [::]:80 [::]:0
Run PlexConnect.py again. No change. Still blocked port 80. ARRGHHH
Then searched for the process which runs the SMSvcHost. Identified the Net.Tcp Port Sharing process. Killed the service. To kill this one a lot of other services had to be killed. I think this can't be correct. After that e.g. netstat isn't working any more

Tried to kill http with "net stop http". Also for this some other services had to be killed:

After all http can't be stopped.
Trying to run PlexConnect.py again. Same result. I'm really go crazy.
I've read nearly everything in the board and wiki concerning 10013. Also asking google tells me noting else than the board or the wiki.
I have absolutly no idea what I can do now.
I will try to uninstall Python again an install 2.7.5 instead of 2.7.6. This version was mentioned as the best one for Python. Maybe this helps.
Hope some of you can help me.
thx a lot in advance
Alex