ok - couldn`t stop tinkering...
gc3e054b on nand, g5207357 on sd card
got freezes on both.
somehow gc3e054b started working from nand - didn`t change anything. g5207357 boots occasionally.
I tried it countless times, almost all the time since my last post. 40 minutes of constant rebooting, turning the thing off and on, rebooting from sd, rebooting from nand, rebooting with and without usb attached.
This is so weird...
gc3e054b booted ~60% of all tries, g5207357 10% at most. USB connected or not does not seem to make any difference, not if connected or not and neither which port.
one last edit before heading to bed:
It doesn`t always hang at the same point. Sometimes the splash screen will stay forever, sometimes it hangs right after the splash screen with a blackscreen.