The network connection was lost while downloading the map

[MatthewNorton]

I can't seem to join any servers! Are they all the same?

The download was almost complete, but then I got an error message saying the network connection had been lost.

[kamnet]

Either your connection is too slow or the server's connection is too slow. The server host has the ability to change the time frame for downloading maps and other info before it times out. But if you're not the host and can't get in touch with them, you're out of luck.

[mauried]

The line that has to be changed is in the network section of openttd.cfg.
This is the default
max_download_time = 1000

Ive found that increasing it by a factor of 20 to 20000 solves the download timeout problems.

As Kamnet says, this has to be done on the Server.
It might be a good time to permanantly increase this value in OPENTTD as maps can be a lot bigger now than in the original TTD , and large maps with lots of towns and industries on them can take a long time to download.
The problem gets worse if people are using mobile phones as hotspots to access the Internet, as this can dramatically slow the download rate.

[Eddi]

it might be a bit naive to think that this solves the problem, because if the download takes exceptionally long, the server either needs to be paused for a long time, disturbing the other players, or the client needs to catch up with fast forward to the server after downloading the map, which might exceed CPU power, and leave the client kicked anyway because it lags too much behind