Hi,
It's realy not my day.
I wrote LibreELEC-imx6.arm-7.95.2-sr-3.14 and LibreELEC-imx6.arm-8.0-7.90.005.1 to two different SD cards and before booting I added the parameters for serial console to U-Boot and after that my CuBox did boot without problems.
Once again sorry for the inconveniance.
Btw. LibreELEC-imx6.arm-7.95.2-sr-3.14 works like a charm, thank you for the great work.