Ubuntu 16.04 stuck after decrypting drive–probably X not starting because of NVIDIA drivers and dist-upgrade (systemd-logind: failed to get session)

For a while now, I’ve had NVIDIA 418.56, CUDA 10.1, and a 4.4.0-148-generic kernel.

I might have caused issues when I ran dist-upgrade or similar recently; after decrypting the drive, it gets stuck there. This is not an issue with the decryption, as I’ve also tried recovery mode and logging into the root shell, and I could work with the shell right after it asks for the decryption credentials.

Running startx from another tty did not work, so I thought that I needed to reinstall the driver. I upgraded it to 418.67 and rebooted (confirmed by nvidia-smi), but the GUI still would not boot.

The Xorg logs are shown here. An error is seen:

(EE) systemd-logind: failed to get session: PID 1214 does not belong to any known session 

Where do I go from here? I’ve searched about the topic, and the posts were mostly from a few years back, involving Arch Linux and Bumblebee.