I can try to use wireshark when it is failing, but what I remember there is normally so much traffic that it is really difficult. There are sometimes testsuites running which are testing the worst...

@Clive_A There are two networks which cannot talk to each other (because of firewall), but it is possible to connect from outside network to both networks. The following diagrams show the two...

Telnet is failing after some time. The web interface is still working. It is not possible to connect via telnet (either blocking or connection rejected). Enabling/disabling telnet has no effect....