Downloading the launcher from https://forum.wow-freakz.com/WoW-Legion-client-t452441.html (last updated: 27-06-2017 17:34) and replacing it with the 1 in the non-working client doesn't work for me... ...
Replacing the new launcher doesn't help. I don't have proxy.
Also when I untick the read only attribute of the data folder and click apply and I re-open properties of the folder, it is still ticked ...