The PC with which I have network troubles has Windows 7 Embedded (WES).
From this PC it's possible to connect to other machines and devices through different ports, but this machine doesn't allow incoming connections (e.g. for ping or VNC). Most obvious would be is to check the firewall settings, but this isn't possible since there is by default no GUI present for the firewall, although there is the Windows Firewall-service. For testing I disabled this firewall and tried to connect again. Same result.
It is possible to add extra packages after the installation of an embedded Windows, see: http://reboot.pro/topic/16383-windows-embedded-standard-7-add-packages-on-a-live-image/
Firstly I added all the packages which seemed networking/firewall packages, but this didn't help. I reached the situation where the firewall was present in the Control Panel, but without buttons to interact. Then I added all other packages (excluding the extra languages) and it resulted in the situation that Windows wouldn't start anymore.
After this I went back to a backup of the OS and stopped the Windows Firewall-service again.
Thus, the question is to make sure incoming connections to this machine are allowed.
Note: Security (and having a firewall) is no issue here, since this is about a network separated from the internet and company network.