Error prevents adding machines to MAAS - 'no datasource found'

My primary question is how to debug this? I have seen this problem twice now, and both times using Ubuntu 18.04 (instead of 20.04) made the problem go away. I also tried adding the address of my MAAS instance to the DNS entry for the boot net subnet, but that did not help.

Details.

I have seen this on MAAS 2.9 and 3.0:

root@maas3:~# snap  list 
Name          Version                 Rev    Tracking       Publisher   Notes
core18        20210722                2128   latest/stable  canonical*  base
core20        20210702                1081   latest/stable  canonical*  base
maas          2.9.2-9165-g.c3e7848d1  12555  2.9/stable     canonical*  -


root@plano101:~# snap list 
Name               Version                  Rev    Tracking         Publisher   Notes
core18             20210722                 2128   latest/stable    canonical✓  base
core20             20210702                 1081   latest/stable    canonical✓  base
maas               3.0.0-10029-g.986ea3e45  15003  3.0/stable       canonical✓  -

In the case of 3.0, I was commissioning 7 roughly identical machines. They were all the same make, same BIOS, but some had more disk than others and some had better CPUs and more memory. But no system was unique. One of the 7 machines consistently refused to commission. It either hung up trying to boot initrd or it would fail with the infamous “no datasource” error. The one that failed was one of 4 identical machines.

I switched to using 18.04 for commissioning and it passed first try.

So again, just to be clear. I am asking for help diagnosing the issue. Are there logs I can/should look at or do I need to use tcpdump?

Is there anything that I should look at on this particular host (now that UB20.04 is deployed on it).