On a different install of the game using the Ineisis patcher and a lesser compression method for the data.pak, it actually get's all the way through downloading files but when you click Play it errors out with "Can't read client.

I've noticed that it doesn't actually create a game executable file to run either, so you have to run the installer again and start over, but we followed the instructions by the letter as far as building the client to DLL instead of exe and so on and I've never seen this particular error before.

means the client DLL cant be found.

That is because it is attempting to read it from the data.pak, but, I am not sure if it has to be added to the pak manually or the Installer/Patcher handles that.