I booted UART normally and I did absolutely nothing else or different. Before this cycle, I tried smaller sdcard (16gb instead of 32gb), fat, fat32, 1gb partition, all MBR (had a plan to try GPT also, but didnt get there), print init_display, usb flash drive (not detected and sdc_update says only from sdcard in help), really lots of these little stuff and absolutely NOTHING solved that MBR header error.
I wanted to try to do both commands 2 times in a row (because of this post Kinhank G1 - #912 by matiws2008), but when I booted UART, on the first try the MBR header error did not appear.
Again, i did absolutely nothing to make it happen. I suspect because i power cycled so much times, that maybe that little booting that it got in all of those tries, that maybe something changed on the internal emmc. Maybe, just maybe, android has to be booted atleast once (because both of my G1’s are straight from brand new box to UART flashing). Just a thought.
UPDATE:
And now for the first time, something has changed.
For everyone reading and in the same boat: MBR header error when flashing bootloader is the problem! Dont know yet how to solve it, since my other G1 is bricked, but try to do both commands two times as in Kinhank G1 - #912 by matiws2008 and also try to reboot/flash/do all over again multiple times as I described at the start of this post.
UPDATE2:
Burning succeeded. Flashing CE onto sdcard now.