Tried it right now from my phone, connected using 4G, and the scan completed w/o problems; maybe there's some caching or something "misbehaving" on your side; notice that I went all the way from www.grc.com frontpage to ShieldUp! using the links, didn't go straight to the scanner page
Thank you for testing.
When I use the same Saumsung S22 phone on 5G data or VPN while connected to my router, I don't have issues.
I tried to go to:
Then click shieldsup, but same result. Blank page for the scanner.