I also experienced the same problem with the 'Send an email' action.
Though I found a solution!
I did see something about an error regarding an 'authentication token has expired' before receiving this issue, after I added the 'Send an email' action to the workflow. The error appeared above the action, but has since disappeared. My guess was that the error was still valid, but the display message for the error has disappeared.
You can resolve this by:
Navigate to your 'Send email' action, and click the '...' > 'My connections' > 'Add new connection'. When I had this error, it was because I had to re-authenticate whatever token is required to sustain the connection. By adding a new connection, it was re-authenticated and emails were successfully sent.
Be sure to check the correct email inbox as well, especially if you are using a demo user rather than your regular inbox.