Overview:
The end-user is receiving an error when attempting to log in: OAuth Error You didn't complete the approval process before it expired. Please try again.
Root Cause:
The URL that SFDC presents has a session ID which expires after approximately 10 minutes of idle time. When a user tries to login after the session id is expired, the system throws the OAuth error.
Solution:
Typically, clearing the browser or device cache fixes the problem.
Related Documentation:
CRM Help Documentation: N/A