Z270 Taichi and graphics card not working

My old Z270 Extreme 4 died on me while in my flight simulator. As I want to use as many of my SSDs, CPU (i7700K), NVMe cards, I bought a Z270 Taichi.
After many experiments and search through the manual, I got CPU, SSDs, NVMe installed.

The current hardware installation is as follows:
Intel i7 7700K
2 NVMe sticks in M.2 Ultra sockets 1 and 2
SSDs in free slots

BUT whenever I try to install my old graphics card nVIDIA GTX 1080ti that worked flawlessly in my old mobo, the Taichi BIOS will not boot. It stops POST with A2 as code on the LED. I tried every PCIe slot, to no avail.
I also tried every permutation of NVMe sticks in the slots; they all work fine until I install the graphics card.
I even tried a different graphics card that I had in my spare hardware box, no luck.

Can anyone tell me what I did wrong or what to do to get a (any!) graphics card to work?
Tx
Volker

Problem solved!

A friend in another forum gave me the tip to connect the graphics card with the monitor (DVI plug). Although I did not believe that this could make the difference, I did as told??and it worked! After so many tries and experiments, the main problem was identified.

Here is my new hardware:

Intel i7 7700K 4.2 GHz (OC)
32 GB DDR4 RAM G-Skill 4 X 8 GB
1 X M.2 NVMe SSD 1 TB for OS and flight sim software
1 X M.2 NVMe SSD 1 TB for sceneries, aircraft etc.
1 X SATA SSD 1 TB for installation files etc., and backup
1 X nVidia GTX 1080ti (short) w. 11 GB GDDR RAM

I am now looking forward to installing the Prepar3D software and the various sceneries and aircraft??and - after a much too long time of inactivity - to ?žfly??again my beloved Spitfire, Me 109E over Europe!

Volker