I'm also having this problem, which stems from an ongoing issues with the Blue Screen of Death. So far I've unsuccessfully tried to register my Roku by doing the following:
-Tried it on 3 devices
-Using 2 different browsers
-Restarted wifi connection for these 3 devices
-Soft reset Roku
-Reconnected wifi settings at least 3 times
-Checked the HMDI cable
I am at the end of my rope where Roku is concerned. I've put up with its BSD shenanigans for years now (not my 1st device) and am considering an alternative solution altogether.
Experienced the same thing with a new Roku Ultra. Got it to work. Here’s how:
Log out of your Roku account on your device. Reboot the Roku, and select 2G when prompted to choose a network. Do a full factory reset by pushing the * on the remote. Then don’t let it autodetect. Instead, manually select your display settings. Then log back in on your device and go to the page to enter the code. This page should load/work now. Use the code on the screen. You can go back to 5G once everything is set up and working. Hope this helps!