It seems that newtmgr runs, uploads something (the progress bar advances until almost the very end), and then produces an “Error: 3”. Quick search didn’t produce useful clues, so I’m asking for help.
[13:34] jwr-macbook-pro:/opt/nordic/ncs/v1.5.0/nfed/samples/blinky>~/nordic/nrf9160-tools/newtmgr -c serial image upload build/zephyr/app_update.bin
63.70 KiB / 65.69 KiB [===================================================================================================================================================================================>-----] 96.98% 15.82 KiB/s
This is on a Mac (11.4), I tried with ncs 1.6.0 and then downgraded to 1.5.0. I did install the serial driver and gave the kext permission to load.
I should probably mention that this is the first time I’m actually bringing up the board, so whatever bootloader is on it is likely quite old.