Sorry I've spent 12 hours trying to get this to work so I've tried a lot, difficult to summarise.
Tried the HMD on 3 different PCs with fresh SteamVR installs, error 108. Main PC has 2080Ti, another has a Quadro P5200.
Tried bypassing the link box, cabling direct to the PC, error 108 still.
Re-seated all cables, unplugged all devices and re-plugged.
Re-installed SteamVR, GPU drivers, run SteamVR as administrator, run vrmonitor.exe as admin, error 108.
The weird thing is that if I disable Direct Mode, the HMD light goes green and the HMD shows the windows desktop, it's recognised as a monitor. It's also recognised in SteamVR USB area as being there on all ports. But SteamVR says Headset Not Detected.
I don't know why it's flipping the photo but you can see here with Direct mode disabled, there is life in the HMD but if I enable direct mode, the LED stays red and still error 108.
This is with Direct Mode enabled, it's connected on all ports but still error 108.
If anyone is able to offer advice, please keep in mind that I've connected this up to a separate fresh PC which should rule out dodgy USB ports or driver issues.
One final possibly relevant detail, I did connect up a brand new Vive at work today using my Steam account in an isolated environment, then when I came home and tried my own Vive, this happened.
So after 12 hours of losing my mind with this, I fixed it.
Possibly a unique situation but someone else might have the same problem.
Around 6 months ago I installed the GearVR lenses into the Vive and eariler yesterday I tried to edit the distortion in E:\Program Files\Steam\steamapps\common\SteamVR\tools\lighthouse\bin\win32 using Lighthouse_console.exe and I think I might have tried to upload an incorrectly formatted JSON file, but I thought I had corrected it by reuploading the correct formatting!
But this morning I've restored the backup of the original config and uploaded that, and immediately it picked the headset back up.
I'm properly confused why this happened though, I thought the HMD would be seen in SteamVR regardless of lighthouse status.