Rock PI 4B V1.4 does not boot when NVMe SSD is connected

Dear Guys,
I have following configuration:

Rock PI 4B 4GB v1.4
Rock PI 4x - M.2 Extension board v1.4
ACER M.2 NVMe SSD for Rock Pi4 - 1TB

I have successfully installed the u-boot image to SPI flash following the Method Two: Write image to SPI flash using mtd tool.
I can boot from µSD without problems as long as the NVMe SSD is not connected.
I can access the SSD when connected via an USB adapter.
When I connect the SSD to the extension board, the green LED is on, the blue one stays dark.
The same is when I connect the SSD directly to the Rock PI 4 board.
Any glue what is wrong?
Thanks!
Herwig

How was formatted your NVMe?
There is possibility, that uboot in spi looks for boot sequence on nvme, found something similar to what he expect and hangs there

I installed the ubuntu 18.04 server image with Etcher and dd, and I tried with ext4 on the empty drive.
But the system neither starts from µSD nor from something else when the SSD is connected, even if there is a system on the SSD or not …