I tried the -r3 option: retry 3 times as follows
newtmgr -r3 -c serial image upload build/zephyr/app_update.bin
and this works with a higher probability than with no -r option, I am thinking that maybe the high speed and cable combination is causing issues.
Can I use lower baud rates? I tried to change to baud=115200, but this did not work: the download was not even starting.