This is a problem related to the rebranding of RStudio Cloud to Posit Cloud.
You can fix it by going to the User Panel (click on your name/icon in the upper right corner of the window) and click "Authentication".
On the page that appears, for your GitHub login:
- Uncheck the "Private repo access also enabled" option
- Recheck it and follow any prompts.
We're sorry for the trouble this has caused - we are investigating and will update with more information, but this should get you unblocked. Please let us know if you or your students continue to see any issues.