I’ve already fixed this, but in case anyone is having the same issue:
I’ve upgraded nVidia driver from 358.16-1 to 361.28-5 (I previously downgraded my kernel from 4.4.x to 4.3 due to issues with sleep, so drivers had to be downgraded as well). After reboot, lightdm would keep crashing and restarting and crashing again, so I had to boot from live USB in order to figure what went wrong.
Turns out that “libGL.so.1” was absent from its usual place (and apparently that was caused by the upgrade). Issue was fixed by making “libGL.so.1” a link to “nvidia/libGL.so.1.0.0”.
(On the unrelated note, why is lightdm set to always restart with nothing to stop it if it restarts too much — like once every two seconds? You can’t even get to a text terminal when lightdm starts crashing repeatedly. I mean, I’ve added the “StartLimitInterval” and “StartLimitBurst” options to the appropriate file, but not having them there by default seems like a bad idea).