I just upgraded to Ubuntu 15.10 and switched from Bumblebee to Nvidia Prime. I'm currently using the 355.11 driver from https://launchpad.net/~graphics-drivers/+archive/ubuntu/ppa, but the problem also occurs when using the drivers in the official Ubuntu repo.
Prime works as expected, apart from the fact that I need to reboot in order for it to switch cards. If I logout and try to login again after switching, it just keeps showing me the login screen after I type in my password. This happens regardless of whether I switch from Nvidia to Intel or from Intel to NVidia.
Here's my /var/log/Xorg.0.log after I switch from Nvidia to Intel and try to login: http://termbin.com/n75a
When I go to TTY1 and do "prime-select nvidia" (the driver that was loaded during boot), I can login again at TTY7.
This question seems similar to ubuntu 14.04 - nvidia prime only works on restart (logout-login doesnt work), but the only answer there is to install Bumblebee...
Does anyone have an idea on how to solve this?