Are you connected in this psql session via tcp or unix domain socket?
Right, got me again. That was a Unix-domain socket.
When I do SHOW tcp_keepalives_idle; from pgAdmin it shows me 120, which is correct.
Thanks for clarifying that.
So that means I still don't know why the connections are breaking.
I know that this could be anything, in any case not due to the postgres server.
Our ISP has inspected the network traffic and indeed found empty TCP ACK packages being sent back and forth to/from the user's IP, supposedly keepalives.
I contacted the user and doublechecked their statement that they only have the issue when running long queries. Turns out that this is not the case. The connection also breaks on idle query windows only then they just reconnect so it's not a problem.
The user now indicated that they can work around the issue by creating a table as a result, instead of simply selecting the data to be displayed in the client.
So we decided to cease our efforts to fix the issue.
Thanks a lot for your help!