Unable to flash Aerohive AP121

I've tried to copy the file openwrt-18.06.4-ar71xx-nand-hiveap-121-squashfs-factory.bin with tftpd64 (Windows 10) via pl2303hxa (usb2serial) cable and over serial port (HP notebook). Both attempts give me a kernel panic similar like the one described at https ://bugs.openwrt.org/index.php?do=details&task_id=1264

I'm following the steps posted in this https ://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=10d11859dfbad0b4482d1a50b20ef91eeeeca2d7

When I tried to flash the file openwrt-ar71xx-nand-hiveap-121-squashfs-factory.bin it took much longer to nand write the file in comparison to the former. But when I reset it boots with the error: Bad magic number.

What am I doing wrong here?