This looks like your device/platform is not recognized correctly. I had sth. like that on my own built images in the past due to update mismatch in my kernel/feeds tree for a different device.
So it could be that there went sth. wrong during the build process from build bot? oO
But that has to be juged be a more qualified board member here.
So just a small bump instead of a solution. Sorry.
What bugs me most is I have the same problem with some newer AR300M. I could upgrade a previous batch through their U-Boot. But not a newer batch. Such regressions are awful. Any recommendations which small low-cost router to buy as an alternative then?
this has been handled on the master branch, see commit below. If you want to update it now you either have to backport some commits and make your own image, or just flash a snapshot image. Version 20 will have all these fixes in the new ath79 target.
For AR750S the story is a little more complex...
Read the commit messages and comments carefully, there is an issue with kernel size, you may have to update the uboot image.
If you update u-boot using openwrt make sure to save a backup of the original, and after writing to it verify with checksum of the mtd partition (probably /dev/mtd0) before you reboot or there is a chance you end up with a bricked board.
Just created an account here to let you know that my AR750S is rolling 17.04 without any problems, I sysupgraded few hours ago just after I saw this thread to check if I would've been able to upgrade form 17.03 or not.
Still on ar71xx though. But it seems still stable. I use it as my main router + wireguard VPN entry.
I knew for a while it would get to ath79 before 20.0 because of that kernel size issue (i've been following openwrt and gluon git in fear that the AR750s would be dropped)so mine has been 24/7 since 17.01 and sysupgrade each time until now.
All that to say I find it strange you weren't able to install the latest 17.04
I do own a AR300M, I will check it runs 17.04 later today, I keep you posted.