"first obvious thing would be to downgrade to the "official" firmware again and see if it works there"
I just tried putting P3_1.02 and have the same issue. (good point thought) I kind of figured it was not the VOSIM OS. I'm wondering what type of hardware issue might cause this.
"Can you enter the bootloader mode with pressing the Menu button on power up?"
Not on initial power up (I.e. plugging in the DC power). I have to hit the reset button first (and hold menu or whatever) to get to boot loader.
So if I plug in power and I don't hit the reset button the screen stays black and the red led next to the microprocessor never blinks. The blue led and yellow LED stay lit.
[edit] new datapoint, I went to do the DFU style boot loader update where you tie boot0 pins with vcc on back side of the main PCB. I jumpered the right most two boot0 pins and powered on by connecting the DC power... and now everything is working normally.....
To be clear, I did not update in DFU mode yet, just jumpered those 2 pins and plugged in power and it consistently boots fine. Super weird.