As it stands, 99 attempts out of 100, I am able to successfully download the ZIP from your account, even under the below conditions. So my recommendation if you ever see this error would be to wait one minute and try again.
I notice that it is theoretically possible for this to occur, although I can only make it happen if I delete all copies of all download packages from the server, forcing it to regenerate when requested (even so, regeneration normally takes 3-5 seconds). It appears to be a bug in PHP where it randomly throttles itself for a few seconds on certain commands especially when repeated in a loop. Ten iterations of the same command can take 0.001s, but then the next iteration of the same exact code will take .1s. If it throttles too many commands in a row like this or worse, the script can time out, which is essentially what you are seeing.
I'm not sure if there is a way to resolve the situation fully. The bug has existed in PHP since at least 2002 and they have never offered a solution or a workaround. In the mean time, I will just see if there is a way to make regeneration more efficient.
It is strange that you would encounter this situation at all, let alone so frequently. It might be possible to cause a race condition during regeneration if you double-click the download button before it grays out. Have you been having problems with your mouse double-clicking unexpectedly? I recently replaced my mouse for doing that.