This issue can be caused by an invalid authorization being stored in Gmail.  The user should attempt the following steps to remove the existing authorization and then try again:
  1. Login to Gmail.
  2. My Account > Sign In & Security
  3. Apps with Account Access.
  4. Remove access to Luminate Online.
Once completed logout and back into the participant center and go through the process of importing contacts.  If the issue still occurs then contact support and provide the time of the error and the cons id of the user that experienced it so it can be further investigated.