I encountered a similar issue while swapping hardware (from RPi5 v2025.3.3 to RPi5 with a larger drive which was flashed directly using the raspberry pi imager)
After the fresh install on the new hardware (old hardware taken offline), I could get to the Observer page, but Supervisor would not load.
This confirmed that the network was fine, but obviously something under the covers was not functioning correctly.
to make a very unsophisticated from-the-hip style troubleshooting story short, I ended up running a core rebuild from the cli which actually failed and produced an error. I then ran a supervisor repair, since I didn’t have anything to lose, and that was successful. 8123 began to respond. I rebooted and it allowed me to restore from backup. All add-ons/devices seem to be working without issue.
rebooted several times since just to be sure.
I wish I could provide better detail and dig into logs and all that cool stuff, but since it’s working, I need to move on to other life tasks 
who knows, maybe it’ll bork and I’ll be forced to put more time into it. we shall see.