Could be because you do not run a rocksolid internet connection.
You simply do as instructed "please obtain a new copy of the program", but if no changes are made to the connection chances are you will just get the same error again.
Here's a quote from the big help section, sure this about playing the game in general, but the same thing can go to you when you just try to download an installer.
If you do use torrent though it always checks up on the downloaded files and that they are correct. That's why torrents are suggest lots of times around here.
How's your internet connection?
Sometimes the internet connection of the users trying to play the game might be weak. This can cause you to not being able to connect or staying online for a longer time, many people might start blaming the game or server for this when it in fact might be themselves.
Here's a few things you can check:
1. Are you even connected to the internet right now?
2. Are you running any downloading/streaming programs?
3. Are there any other computers/devices using the same internet?
4. Are you currently using a wireless connection, if so, are you getting good signal?
Sometimes the internet connection can time out for various reasons and then jump back on again, so be sure to try a few times first.
Programs in the background
Downloading things or running streaming programs eats your bandwidth, and this bandwidth is needed in order to play the game, especially if you're using slower speeds. You can start by shutting off things like downloading programs and internet browsers. Maybe even IM programs such as MSN and AIM and try to play again.
Turn off extra devices
If you got more than 1 computer or other devices also using the internet, try turning them off if they're not in use. This will give you more bandwidth to spare.
On a wireless connection?
If you're using a laptop you might be using a wireless internet connection. These can sometimes loose their signal to the internet source. So try placing yourself closer to your modem, this might give you a stronger signal.
