I decided today that I would transfer my Pi4 HA installation to a new SD card. After putting in the new SD card and going through the onboarding, I’m met with a blank screen (see screen shot). Refreshing the page gives me a 404 error.
Just to make sure I wasn’t crazy, or my Pi was the issue - I installed through a VM and I’m getting the exact same result.
I did a Home assistant os on a Rpi4 a few days ago with this image: hassos_rpi4-64-4.12.img.gz and all went well.
Do you have a proxy server?
I have a proxy server for my network and if I forget to allow the Rpi4 to bypass the proxy server I see what you are seeing.