I am using Lenovo Thinkpad P50 with dual booting system (Win 10 and Ubuntu 16.04), Wacom Pen support and multitouch screen. After I enabled the propriety nVidia driver (361.42?) and rebooted, I got the login loop problem. Every solution I can find would suggest to go to the console or the TTY mode to enter some commands, but in my case I cannot even switch to the TTY mode -- ctr+alt+F1 doesn't work! BTW, I was trying to make the second monitor work with the nVidia driver for the nVidia Quodra M1000M card, but no luck so far.
Update: I finally got into console by chance (don't remember what works).
Tried: all the solutions on Graphics issues after/while installing Ubuntu 16.04/16.10 with NVIDIA graphics. nVidia-361 abd nVidia-364 drivers didn't work at all. UEFI has been turned off. I also uninstalled Unity (may not be complete) and installed GNOME 3.20. Tried deleting the .Xauthority file. Using Linux kernel 4.6. See comments on the answers below.
It only works when I uninstall nvidia drivers, but I need the nvidia driver for external monitor (so far no luck to use the external monitor).
Any suggestions?
One error before unstalling unity.
openConnection: connect: No such file or directory
cannot connect to brltty at :0
upstart: gnome-session (Unity) main process (2642) terminated with status 1
upstart: unity-settings-daemon main process (2632) killed by TERM signal
upstart: logrotate main process (2492) killed by TERM signal
upstart: bamfdaemon main process (2545) killed by TERM signal
upstart: Disconnected from notified D-Bus bus
upstart: unity-panel-service main process (2647) killed by TERM signal
upstart: job indicator-bluetooth failed to stop
Also, when I see the login loop happening, there is a flash of window popped out saying "Unknown tablet connected. The Wacom Inc. Pen and Multitouch may not work as expected..."