I was able to verify that the box is ok by flashing another download of the stock Android rom, so I tried flashing the Linux image again.
The log says "16:05:43 Error: failed to stat config.ini, err=2" repeadetly, console output is:
karstein@Pallas ~/A5XMax/a5xmaxplus_linux_flasher $ ./flash.sh
Downloading bootloader succeeded.
Writing gpt succeeded.
Write LBA from file (100%)
Upgrading loader succeeded.
Reset Device OK.
karstein@Pallas ~/A5XMax/a5xmaxplus_linux_flasher $
but the box does not boot when powered up. Monitor is not turned on (ie no video output) and the red LED in the box is the only one lit.