16.04 upgrade login fail

I upgraded from 15.10 to 16.04. When I try to login, an error report message appears briefly, and I retu to the login page.

I've tried installing and using gnomes he'll (no change), and reverting /etc/X11/xorg.conf (no change). lightdm is still working, so I'm confused.

The log file (Xorg.1.log) is below, but I can't make out anything wrong. Anyone see anything I don't? I'm going to try updating drivers next.

http://pastebin.com/NTwcebvN

Update: apt-get is now reporting that protocol "http" is not supported or disabled in libcurl. Mentions of this on google mostly blame windows.