..had the same prob (second time in over two years that there was a prob)..
Tried to update several times and did not work, file was already downloaded and seemed complete.
Tried to execute the update, got the error "...not an X86/64bit file..".
Deleted the update file, restarted the client, downloaded and patched fine.
Looks like the first time it was not downloaded correctly and nothing in the updater to check the downloads integrity.
My problem is surely not the same as the "won't download at all", but also got the 404 error after downloding strangely