I have identified the place where it is hanging on (red screen). Oh, how good it is I have added color identification/diagnostic .
I have also theory how to improve it, but it is about timing - number of waiting cycles when the C64 and FPGA is powered on and the SD controller is initialized.
When FPGA is loading firmware it takes for a while. The C64 power on sequence takes a while as well. And it seems to be there are differencies in this timing between C64 boards.
I believe that I have to wait longer time in my starting code - boot Ultimax cartridge.
1) I have no such C64 machine to test it.
2) The code is part of FPGA firmware.
If you need to change it, you need reflash FPGA firmware, you will need USB Blaster Rev.C programmer + Altera Quartus Studio installed on your PC.
What to do ? I can only check the theory in reverse way, I will set lower number of cycles and my expectation will be I will end in red screen as well.
I strongly believe I will solve this problem. Only confirmation will take a time.
Thanks all, perfect inputs and good night.