I have a exactly same kind of problem and tried everything (DNS change,reset DNS, Register new IP, restart game client and computer) , but nothing can solve this problem.
Until I right click on GW2 Shortcut and choose “Run as administrator” then my game patching is starting to work again.
Edit : I recently found that this problem come from my antivirus and google DNS problem combined. but I’m not sure about “Run as administrator” choice.