I wanted to reset my board completely and tried the procedure with holding up, down and select. After this I got a white screen - I think this is ok?
I took TinyBooter Updater - it says “Sent file & Memory area content (address: 0x00000, size: 131072 bytes) match exactly !” in the end - so it seems to me to be ok.
Problem: The screen remains white! What should I do?