Dear Tom, I tried every step repeatedly, tried to flash FW directly through OpenOCD and also through STM32 flash many times, tried to reinstall drivers, too - still the same behaviour: After the cold start the display is off and the i2cdetect output shows 0x56 again. I spent lots of time with this issue, I will wait for the response from the seed support. I hope, there will be solution.
Thank you, Tom.