I experienced a similar issue just now while trying to install Ubuntu 18.04 LTS on a system with two Titan RTX cards. The install process went fine - I selected third-party drivers to be installed as part of the process and provided the UEFI secure boot password, etc. The nvidia driver that was installed automatically was version 430.
After rebooting, the system came up but I was not able to login - got stuck in a login loop. Shutting down the system and waiting a few minutes before rebooting would result in the system somehow deciding to go with the default neuveau video driver and would allow me to login but the nvidia-smi command would still result in errors indicating that the nvidia driver was not functioning. After reinstalling the system a few times with different kernels and all sorts of other attempts at installing various nvidia drivers going all the way back to 410, I was at a loss. None of the suggestions in this thread or others that I found on the web worked at all.
The solution that worked for me was very obscure. Just by trial and error, I was going through BIOS boot settings on my motherboard (ASUS WS X99) and noticed that there was a UEFI option in the secure boot section that by default had Windows selected. I changed that to “Other OS” and rebooted - I was able to login and ran nvidia-smi - all looks good.
I spent about 6 hours tracking this down. Hope this post helps someone with the same or similar issue.