Armbian_21.05.1_Nanopiduo_focal_current_5.10.34.img boots (updating via apt upgrade also causes it not to boot any more)
Armbian_21.08.1_Nanopiduo_focal_current_5.10.60.img does not

 

When I examine the SD card it hasn't expended from image (makes sense since the heartbeat never starts).

grab a USB UART adapter to get logs from serial console.

So on a normal boot I can putty into Duo using USB serial. Are you saying I need adapter that uses debug pins? i.e. https://www.cnx-software.com/2017/10/30/nanopi-duo-quick-start-guide-ubuntu-breadboard-mini-shield-msata-ssd

http://wiki.friendlyarm.com/wiki/index.php/File:Duo_pinout-02.jpg

I dont know this particular board but I assume you need to use UART0 (DTX,DRX,GND) to get boot logs if you dont see them on assuming UART1.

OK, let me try the easy way with USB UART, but I also found I have a https://www.friendlyarm.com/index.php?route=product/product&product_id=178 kicking around. Give's me a use for it if easy method doesn't work. And good thing I have 4 Duos because 3 are already on breadboards in use.

OK, have FriendlyElec Matrix USB to TTL logging with working image. Let me try broken one and capture.

Damn thing booted Armbian_21.08.1_Nanopiduo_focal_current_5.10.60.img. Let me try this on the board I had an issue with and the same SD card. At least I know how to grab boot log now

OK, found root cause. I had a board wired up with all kinds of test devices, loopbacks, etc. My bad, sorry about that. I can fire up debug console now as a result. Still not sure why other version would boot though.

@WernerOK, I finally figured this out and it's not my wiring job  What's happening is the Duo uses DRX pin (debug receive) for PWM, so if I wire up a LED it will not boot. If I remove that wire it will boot up. After it boots I can connect wire and it will work as expected, so something changed in Armbian_21.08.1_Nanopiduo_focal_current_5.10.60.img. I suspect that no one noticed this unless they were doing hardware PWM, I guess I need to disable serial console?

No idea. Give it a try.

OK, so I tried console=none, the getty disable ttyS0 trick, etc. Most of the posts indicate you need to make a change and recompile the kernel. I don't have time for that, but for multi function pins this should work like it did in the previous releases.