Thanks very much for your response.
This is my first day working with Armbian so it is still something of a learning curve.
I took your advice and set up the build environment and built images for CT with combinations of Current and Legacy versions of the software for Buster and Focal as supported by the tool.
None of these versions would boot correctly, failing in the same manner as the version on the Download page for CT: "Armbian_21.05.1_Cubietruck_buster_current_5.10.34_xfce_desktop"
These images all have "U-Boot 2021.04" in common. (U-Boot 2021.04-armbian (May 06 2021 - 17:01:36 +0000))
I don't know how to proceed further with debugging, in particular checking for file sizes as you suggested.
I have looked through the documentation Developer Guide / Building Armbian and User Configurations, and through
the build files system, but it is not obvious where this information might be, or how to re-configure.
Obviously I am missing something basic...
I also explored the archive software a little more.
The most recent version I could find there was: Armbian_21.02.3_Cubietruck_focal_current_5.10.21
dated March 2021. This software booted correctly, with "U-Boot 2020.10-armbian"
I also tested several earlier versions with U-Boot 2019.04 and 2019.01 and they also booted correctly.
So I thought it might be worth looking at the environment settings for the different U-Boot versions.
Looking through the printenv output for the two recent versions of U-Boot, there are not a lot of obvious
differences in address settings.
One difference I noticed is the value of the "fdtcontroladdr" setting, as shown below, but I don't know
if it is telling us anything useful.
I am happy to spend some more time on debugging if I can get some pointers in the right direction.