This is the result of buster-backports being removed from the main repository server: https://backports.debian.org/news/Removal_of_buster-backports_from_the_debian_archive/

Confirmed this is now fixed.

I've traced the issue back to a USB device that creates an Ethernet interface on the OS. I suspect that the issue was caused by the USB device not being ready by the time ddclient tried to start and bind to configured interfaces, causing the boot time configuration to fail. Given that this is an edge case that would rarely occur, and now that I have replaced the USB device with an actual Ethernet device, I'm happy to close this bug report.

I've done more investigation of this issue today, and I believe I understand the cause.

I haven't had the chance to test this, but the following should be sufficient to replicate the behaviour:

Maybe making the size of the ramdisk configurable via CLI would be wise? I feel that there's enough variation in hardware configurations out there that hard-coding a value would cause problems.

PR submitted: https://github.com/vyos/vyos-1x/pull/2746