Okay, for some reason which is entirely unclear to me, flashing worked now using the supplied flashall.sh - after 20 unsuccessful tries. Something is not so stable here... But anyway, it works now.
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I’m glad to know that you were able to recover the functionality of the board.
Sometimes there are problems while flashing the board due to the stability of the USB ports, try to change the cables (this could help). In some cases there are USB ports that do not supply the required power for the board so it lost connection and the board reboots constantly, my suggestion would be to use a power supply in these cases.