yesterday morning their servers were having issues, by lunch time it was fine, managed to update my Mac mini server and the other Macs just pulls Big Sur image directly from the mini without a problem

loading as in? boot screen? if boot screen, use another device to ssh into the device to issue reboot command. one of my Mac was stuck at the boot screen for quite some time, which it was doing nothing in the background. Reboot and the installation completes just fine