I have found the solution. The bus number must be 1, not 0 as explained everywhere. This will create the device /dev/spidev1.0 instead of spidev0.0 and applications must be adapted. But with this modification I can access the SPI device.
So here's the relevant overlay configuration in /boot/armbianEnv.txt:
overlays=spi-spidev
param_spidev_spi_bus=1
And the SPI interface can be used only under this path: /dev/spidev1.0