What should I do when “An error has occurred” when downloading Lion?

Question

I'm trying to download the new Mac OS. When click install, the icon pops onto the dock and says Waiting...

After about 5 minutes, it disappears and in the purchased category of the Mac App Store, next to install it says:

An error has occurred

Why is this happening?

Edit: Originally I could not even get the download bar in the purchased section until I reconfigured my router slightly(I'm using a BT Business Hub). I am now able to get the download bar. However, I still get "Waiting..." on the icon and after about 3 minutes, I receive the same error message:

An error has occurred

I still believe that it is a router problem and not the app stores' servers. Has another been able to fix this yet?

Here is a screenshot when I click the install button:

And here is a screenshot after about 2 minutes: