KG reported me that when he typed very long messages into GT's chat the server would disconnect him for half a minute.
That sounds like a curious symptom to investigate at. I found out that freeciv's chat messages are capped at a size of 1536 bytes, which is meant to exactly fit into a single TCP packet. The problem resided in the GT script / binary interface, which was configured to accept payloads into a buffer limited to 1024 bytes. So, when a text message longer that 1024 characters was fired into the chat console, the freeciv-server binary would segfault instantly and get restarted after some seconds by a watchdog, causing a temporary disconnection of the user lasting about those 30 seconds reported by KG.
I have increased the interface buffer to 4096 bytes, which provide the full space for a 1536-bytes chat message, which is anyway internally capped at 1436 because of overhead transporting the message's headers (defined to 100 bytes).
So this solves the problem.. Happy chatting :)