But one stupid question: Is /flash definitively the correct directory on the INTERNAL side?I mean, have a working LE on SD card and with that I copied the correct dtb.img now into /flash to fix the internal LE, which I used the MXQ 4K dtb file for, but the screen stays still black, no boot up at all.
No, for internal you should use these commands (remeber to execute them in folder where you have device tree as dtb.img or replace the name in script):
In case you wondered what the upgrade_step is for, I noticed that some bootloaders don't allow writing to some internal partitions unless you set it to 3.