This seems to be similar to Ubuntu gets stuck in a Login Loop and Ubuntu Login Loop but nothing suggested at either of those two has worked.
I get the login screen. Whether I attempt to log in as myself, another user, or the Guest user, the screen goes black momentarily and returns to the login screen.
Here are the contents of my ~/.xsession-errors:
Script for ibus started at run_im.
Script for auto started at run_im.
Script for default started at run_im.
init: gnome-session (Unity) main process (2548) terminated with status 1
init: unity-settings-daemon main process (2538) killed by TERM signal
init: Disconnected from notified D-Bus bus
init: logrotate main process (2446) killed by TERM signal
init: update-notifier-crash (/var/crash/_usr_lib_ibus_ibus-ui-gtk3.1001.crash) main process (2495) killed by TERM signal
init: xsession-init main process (2535) killed by TERM signal
init: hud main process (2542) killed by TERM signal
init: unity-panel-service main process (2553) killed by TERM signal
init: window-stack-bridge main process (2475) killed by KILL signal
As I noted, I tried pretty much everything I found at the referenced questions and nothing has worked - changing permissions and ownerships on .Xauthority and .ICEauthority, installing linux-headers-generic, reconfiguring/uninstalling/reinstalling lightdm, etc.
I'm kind of at a loss on how to proceed. Any ideas?
(Edit; the next day) One thing I hadn't tried was switching to gdm as described at Ubuntu gets stuck in a Login Loop. That was no help at all. After running dpkg-reconfigure gdm the login screen did not change at all, so I tried rebooting. After that, I got only a couple of seconds of display / active keyboard with each press of ctrl-alt-F2, and it took me about 10 minutes to successfully reconfigure for lightdm. I don't know if that's helpful to anyone reading this....
(Edit) Here is the output of sudo lshw -c video:
*-display
description: 3D controller
product: GM107GLM [Quadro M1000M]
vendor: NVIDIA Corporation
physical id: 0
bus info: pci@0000:01:00.0
version: a2
width: 64 bits
clock: 33MHz
capabilities: pm msi pciexpress bus_master cap_list rom
configuration: driver=nvidia latency=0
resources: irq:16 memory:dc000000-dcffffff memory:b0000000-bfffffff memory:c0000000-c1ffffff ioport:e000(size=128) memory:dd000000-dd07ffff
*-display
description: VGA compatible controller
product: Intel Corporation
vendor: Intel Corporation
physical id: 2
bus info: pci@0000:00:02.0
version: 06
width: 64 bits
clock: 33MHz
capabilities: pciexpress msi pm vga_controller bus_master cap_list rom
configuration: driver=i915_bpo latency=0
resources: irq:128 memory:db000000-dbffffff memory:70000000-7fffffff ioport:f000(size=64)
I can find some info on these drivers with lsmod:
$ lsmod | grep i915_bpo
i915_bpo 1306624 3
intel_ips 20480 1 i915_bpo
i2c_algo_bit 16384 1 i915_bpo
drm_kms_helper 151552 2 i915_bpo,nvidia_drm
drm 360448 5 i915_bpo,drm_kms_helper,nvidia_drm
video 40960 3 i915_bpo,dell_wmi,dell_laptop
$ lsmod | grep nvidia
nvidia_drm 45056 0
nvidia_modeset 765952 1 nvidia_drm
drm_kms_helper 151552 2 i915_bpo,nvidia_drm
drm 360448 5 i915_bpo,drm_kms_helper,nvidia_drm
nvidia 11489280 1 nvidia_modeset
So I'm not sure the nvidia video driver belongs?
modinfo returns some info about i915_bpo:
modinfo i915_bpo
filename: /lib/modules/4.4.0-47generic/kernel/ubuntu/i915/i915_bpo.ko
license: GPL and additional rights
description: Intel Graphics
author: Intel Corporation
author: Tungsten Graphics, Inc.
firmware: i915/bxt_dmc_ver1.bin
firmware: i915/skl_dmc_ver1.bin
firmware: i915/kbl_dmc_ver1.bin
firmware: i915/skl_guc_ver6.bin
srcversion: A0AA67E6671AC7D4B380BAA
alias: pci:v00008086d0000593Bsv*sd*bc03sc*i*
[etc.]
... but there is no info returned from modinfo for any of the nvidia.* drivers. I am thinking that the invidia drivers are either corrupted or do not belong, but I'm not sure what to try next. Thanks anyone for any help.
sudo nvidia-settings --uninstall
andsudo apt-get remove --purge nvidia*