I'm also running into this issue on my optimus laptop.

When I boot the laptop with HDMI connected, it boots to a black screen with the mouse pointer visible. Removing the HDMI cable causes lightdm to appear and plugging it back in causes it to go back to the black screen. Logging in with the cable connected causes the laptop screen to flicker and the TV (secondary display) to load a black screen. Unplugging the cable causes the desktop to load. Plugging back in goes back to the flickering screen.

By flickering, I mean that it goes black, flashes briefly like it's trying to turn the display on, goes back to black, flashes, black.....

This occurs after installing the Nvidia drivers and rebooting.

This segment of dmesg output looks related.

[   62.476438] vgaarb: this pci device is not a vga device
[   64.505517] systemd-journald[470]: Failed to write entry (24 items, 694 bytes), ignoring: Cannot assign requested address
[   67.871349] systemd-journald[470]: Failed to write entry (19 items, 556 bytes), ignoring: Cannot assign requested address
[   67.903333] systemd-journald[470]: Failed to write entry (19 items, 525 bytes), ignoring: Cannot assign requested address
[   67.914360] systemd-journald[470]: Failed to write entry (19 items, 476 bytes), ignoring: Cannot assign requested address
...

The systemd-journald entry is repeated ~40 times after the vgaarb one

Does this issue persist under a fully updated Solus 3?