When I go through a nether portal, I would expect the message "Downloading terrain" and then I appear in the nether/overworld. However, when I go through the portal, minecraft loses the connection to the server and I get the message "Connection Lost - Internal Exception: io.netty.handler.timeout.ReadTimeoutException". If I click on the button "Back to title screen" and try to reconnect, I get the same message again. I have to quit minecraft and restart it (the client) in order to connect to the server.

The server is in the same local network as the client, so the connection should be really fine. A friend (he runs the minecraft client on Mac OS) of mine who is connected to the server via internet has no problem when going through nether portals.

This error first appeared in version 1.8 - I previously ran version 1.7.9 (server) and 1.7.10 (client) and did not run into the described problem.