FWIW, the download completed just fine, but the next time I quit playing, it did the same thing it did before, starting a download and quitting at 17kb. It seems like the download is still intact.
Nevermind - trashed the "prepatch" folder and restarted the launcher - pre-patch download started right up. For those curious, the location is C:\Users\Public\Games\Cryptic Studios\Neverwinter\Live\prepatch
Now I think even the patching on startup is failing, because it looks like it's trying to patch and about to download, then the launcher quickly exits and the game starts.
I'm having problems with the patcher that opens when you quit - it downloaded about 87% and froze, so I quit it. Next time it ran (and every time after), it downloads about 17kb and then quits. I've tried "run as admin", but it makes no difference. The version number is the older one, not the one it's attempting to…