So have discussed this, and the technical explanation is:

"Your firmware is a spud" (and preventing the variable from being created)

There is a way around it though, which is to mount the ESP to /boot before it is run, which can be mounted permanently through /etc/fstab

I'm experiencing this too on a fresh install. After installing all the updates, if I install the current nvidia drivers then Solus won't boot anymore, it will get stuck on a black screen.

When I first installed Solus I didn't have this problem.

Mounting the ESP partition to /boot, running sudo clr-boot-manager update, rebooting and installing the drivers worked.

Did you install nvidia-glx-driver-current? You can't boot into the current kernel (4.14) without this driver version, try to boot into Kernel 4.9, if you haven't installed nvidia-glx-driver-current.

Yes that's the one i installed and after reboot it would get stuck on a black screen.
After installing all updates and rebooting uname reported that i use kernel 4.12.
Then i run clr-boot-manager update which gave me the Cannot determine boot device error that brought me here.

I had to install Solus a few times until i had it figured out.

In the GRUB menu, is there a entry with kernel 4.14?
You can also try to install nvidia-glx-driver-common, and boot into kernel 4.9.

Mounting the ESP partition to /boot, running sudo clr-boot-manager update, rebooting and installing the drivers worked.

That changed the kernel to 4.14.9, which was installed with the updates.

Also I'm using uefi so no grub.

Kernel 4.15?

I don't understand what you're asking.

Is this still a problem on 4.15?

Yes, i still need to mount the boot partition else clr-boot-manager update gives an error

sudo clr-boot-manager update
[FATAL] cbm (src/bootman/update.c:L85): Cannot determine boot device

Don't know if related, but last couple of times i installed Solus i had no boot entry for it in the boot menu and i had to add one manually with efibootmgr.
efibootmgr -c -d /dev/sdb -p 1 -L "Linux Boot Manager" -l "\EFI\Boot\BOOTX64.EFI"
Now if i check efibootmgr in Solus it doesn't show the entry i added, but it's available in the boot menu.