One of the reasons could be an antivirus or anything that can interrupt a download. If you have an antivirus, even windows security (default antivirus that comes with the newest windows operating systems) you should try repatching or reinstalling and then repatching while the antivirus is disabled, you can temporarily disable an antivirus without too much hassle.
As somebody said, they don't think they have an antivirus because they're in an internet cafe, that's not entirely true, they could have some security as well, you may not see it if you are not on a privileged user.
This error always happens to some users when we update our game client, which is detected as a false positive by a couple of antiviruses out of dozens.
We will release a fully patched installer soon, which will solve the problem.