18.04 – Nvidia driver installs on Ubuntu, but fails to load

I am running Ubuntu 18.04.3 with an NVIDIA GeForce GTX 780 Ti graphics card, and I want to install and use the official NVIDIA driver. I have previously been successful with this, and have done many installations in this way for years. But with my current system, after installing the recommended driver, it doesn’t load correctly.

Here is what I have done so far specifically:

  1. First, I tried installing the driver via apt: sudo apt install nvidia-driver-390. But after rebooting, I could tell that NVIDIA driver wasn’t actually running, for three reasons: (i) the screen was in a low resolution (it is in a higher resolution when the NVIDIA driver is running), (ii) When I looked in Ubuntu Software Centre -> Additional Drivers, it showed that one of the open-source drivers was selected, (iii) When I run nvidia-smi, it prints the following: NVIDIA-SMI has failed because it couldn't communicate with the NVIDIA driver. Make sure that the latest NVIDIA driver is installed and running.
  1. Then, I tried to disable the Nouveau driver, using the instructions here: https://linuxconfig.org/how-to-disable-nouveau-nvidia-driver-on-ubuntu-18-04-bionic-beaver-linux. Following this, I manually selected the NVIDIA proprietary driver in Ubuntu Software Centre -> Additional Drivers. I then rebooted, but the driver still was not loaded (low resolution, nvidia-smi error).

  2. I then read that this may be something to do with secure boot being enabled. So, I disabled secure boot at the BIOS. However, this did not change anything.

  3. I then tried installing a different NVIDIA driver (nvidia-driver-435), using the instructions here: https://linuxconfig.org/how-to-install-the-nvidia-drivers-on-ubuntu-18-04-bionic-beaver-linux. This seemed to install the driver correctly, but resulted in a few error messages along the way. I have pasted the full printout here (https://pastebin.com/mgBKXczC), but the two error messages are:
    ERROR: Cannot create report: (Errno 17) File exists: '/var/crash/nvidia-kernel-source-435.0.crash' and Error! Bad return status for module build on kernel: 5.4.0-48-generic (x86_64).
    However, this did not seem to load the driver correctly (again, low resolution, and nvidia-smi error). And when I take a look at Ubuntu Software Centre -> Additional Drivers, I see that actually, the nvidia-driver-435 I installed is actually some open source version, rather than the proprietary version:
    (click here for a screenshot).
    So, all I can do is select the proprietary nvidia-driver-390 from before, which did not work either, as I have already described.

I am completely stuck now as to why this is happening, and what else I should try. Can anybody help please?