I'm using MacOS Safari and I receive a "403: Forbidden" error message after signing in to the Central Authentication Service (CAS) and before the Duo Universal Prompt appears.

Duo/Multi-Factor authentication.

Enable automatic time syncing and set the correct timezone. After syncing the time, you should be able to authenticate with Duo. See Apple's article, Set the date and time automatically on your Mac, for instructions.

Incorrect time and time zone was causing the Duo Universal Prompt to return an error.