Hi @eumaios,
Please check the value of agent_hostname in the configuration file /opt/thinlinc/etc/conf.d/vsgagent.hconf. This will need to be set to something which resolves correctly both inside and outside your LAN, if you need to be able to connect from both networks.
There is more background to this here: Why does ThinLinc try to connect to a different IP address than the one specified?
Also, see the instructions here:
Let me know if this helps!