Everytime you enter a server, your computer needs to load the map. From your hard drive to the RAM (which doesn't mean to download it from the server). It means that your com prepares the map to be played (if you havent got it by now, imagine the computer building the map in a plain white room, stone by stone, plant by plant). It loads every layer and every picture.
While this loading time your computer doesnt communicate with the server. So if your computer doesnt finish loading them within 10 seconds (mostly because there are some very big pictures in the map), the server hasnt heard from you for 10 seconds. So you are timed out.
This is a stupid bug and I hope it will be fixed within the next version. As well as the downloading of maps could be done much faster (according to heinrich5991).
void ..