You need Windows for sure to downgrade the dock. In case you have no other computer for this, there are several options:

Of course this requires a windows license. In case you don't got one: you get 2nd hand licenses for < 20 $/€ , and you can legally use the 30-day free period for windows.

Of course you need to install a windows for this, which rises the question were to install it. This are some options:

• • I kept a small Windows partition just for this purpose, so free space on your current SSD, install windows there.
  • Lend or buy (In case you can afford), possibly ebay or otherwise, any cheep, old or new, slow or fast, big or small (64 GB is ok) M.2 SSD. Plug it in (the P1 has 2 bays for M.2) and install windows there, or even remove your old M.2 SSD, which I highly recommend in case you are unfamiliar with dual-boot systems . You are allowed to do this without loosing the warranty (written so in the users manual). 
  • Or in case you can't spend a single dime: backup your SSD (you do backups anyway, true ?) , Install Windows, restore your backup.

Yeah, all the laptops are Lenovo Thinkpads and had originally Windows license (I bought all second hand and it is not that easy to get it with Linux anyway), but it is too much bother. I returned the dock (I had a deal that I could) and bought a Thunderbolt-C to DP cable, thank god my monitors support MST, see details here: https://forums.lenovo.com/t5/Ubuntu/What-Thunderbolt-3-dock-works-well-with-Ubuntu/m-p/5052714

I was hopeful that Lenovo announcing selling Linux machines would result in firmware that fixes the issues. So far that hasn't happened. 

Just a remark. I had so many of the same issues.

There was a chance for any of these things to happen after making a display configuration change, suspending/resuming, undocking etc:
- Nothing (displays work fine)
- One display stays black
- All displays including laptop internal display freeze, which is recoverable only by physically disconnecting the displays from the dock
- Displays flicker on and off rapidly before every one goes dark including the laptop's internal display, requiring a hard reset.

Sometimes the USB hub would just disconnect, ethernet would stop working, etc. Sometimes everything would just freeze.

Recently, I replaced (by warranty) to a new dock. I finally tested it, and now I have had the following behavior with a Lenovo P1 Gen 2 (Quadro T2000), a Thunderbolt 3 Workstation Dock Gen 2 (firmware sub-version 64) and nVidia drivers 455 over Ubuntu 20.04 with kernel 5.4.85-050485-generic:

I can not log-in at all unless I had the Dock connected, for some strange reason. After entering the password, with the laptop undocked, Ubuntu would freeze - the session would not start. (The mouse still moved, but the UI was no longer responsive).
In Windows, everything worked fine.

Kernel 5.4.58 had the same issues. Oddly, in kernel 5.4.56 I could log in, but still had all the issues you mentioned above.

Changing the drivers to Nouveau meant I could log-in, but only one external monitor worked (I have 2).

All of this with Kernel DMA Disabled and running at "No Security" to make everything simpler.

What I ended up doing was:

Changed to nVidia drivers 418 (I lose the smart power management, so battery time is terrible).
Thunderbolt 3 Security at "User Authorization" (for now).

Now, everything works (so far). Suspending works, resuming works, shutting off works (all of these would sometimes freeze).

It's pretty sad that this is the state of Lenovo's state of the art laptops and docks, on Ubuntu which they are supposedly certified for.

We are now sadly considering moving our entire engineering department to Macs in our next upgrade cycle.

Lenovo has kind of made the dock work okay with Windows, but they have done absolutely nothing at all to fix the situation on Linux. I had hoped that they would fix this when they started supporting Linux on some of their computers. I absolutely would not recommend this docking station to anyone.  I no longer run my video through the docking station at all, which is a true shame considering how much the docking station cost me. 

Worse still, P1 Gen 2 + WorkStation dock are UBUNTU CERTIFIED:

https://certification.ubuntu.com/hardware/201912-27612

Maybe we should contact Canonical and ask them to remove the certification, and also for P1 Gen 3.

Has anyboyd tried with Ubuntu 18.04? If it is the same mess as with any other distribution, then yes, we should let them know.

@alon_mayple : Thanks for that hint. A the time when I opened this issue (I'm the original poster), I did not found a certification for the dock (only the P1 ), so unless I missed it, it was certified later.I just opened this :

https://answers.launchpad.net/ubuntu-certification/+question/694734 .

I did read the thread when I first posted but that was a month ago and I have since forgotten. I do not think one has to read all the thread anew before replying again. Anyway, I would assume that Canonical would want you to test with the latest firmware before reporting problems and the newset firmware was not available when you started this thread (I am aware that new versions of the Dock's firmware do not improve things). Thank you for raising the issue with Canonical.