More on this. Again, with help from Esri Canada Tech Support, another registry update seems to have solved the issue for me - for both ArcGIS Online sign in and for ArcGIS Enterprise (portal) sign in. As before, use caution when editing your registry (consider a backup), and I realize not everyone has the admin permission to do so. But this worked for me.
First, in my case, I had to change back the earlier registry change I made (you many not need to do this):
If you changed it, go back to:
HKEY_CURRENT_USER\Software\ESRI\ArcGIS Online For Pro\SignIn
and set the value data to 0 for the SignInFromBrowser key.
Next, try this:
Find the following: HKEY_CURRENT_USER\SOFTWARE\Microsoft\Avalon.Graphics\
Set the value data to 1 for the DisableHWAcceleration key (you will likely need to create this key as a DWORD 32).
This will disable hardware acceleration on the machine, so I don't know what impact it may have on other applications!
When I created this new key, the Sign In window worked in ArcGIS Pro 2.7.2 for both ArcGIS Online and ArcGIS Enterprise Portal. I have not tried it, but if you are worried about the impact of this change, you could change the registry setting, set and sign in to your portals in Pro, and then set the registry value back...
This will revert back to the non-browser sign in method.