Usually we just enter https://cs50.dev URL in the browser and after login in GitHub it opens codespaces but currently. I am trying from whole day and it doesn't work. Even I made a new account for both cs50 and GitHub and tried access this URL but it shows same error, "Setting up codespaces" on a blank white page.
Here is Screenshot of Situation:
Cause of Problem:
I emailed the Cs50 team and Found that it was caused by Network Provider not allowing to accessing cs50.dev or because of some other issue related to this site and Network provider.
The Solution is:
First one worked for me:
The solution is simple use VPN for accessing Cs50.dev.
Change your Network Connection.