Hi, exactly the same behavior after upgrade from wheezy to jessie. I've found out that function awful.util.spawn() which is used to auto start applications during start up of Awesome WM causes X to crash. Commenting out applications to start up in rc.lua is just a temporarily "hack", reverting to awesome_3.4.13-1_amd64.deb also helps;)
Regards -- Piotr Kweclich Możemy być tylko tym, czym jesteśmy, niczym mniej, niczym więcej...