You are seeing this error as a result of changes Google has made recently. To set this right, please follow the steps provided:
Once you’ve followed the above steps, please go to Newton and Re-Auth your account. To Re-Auth your account, go to Settings > Email Accounts > Select the affected account > Click on the account name again. You will be redirected to that account’s login page where you will have to enter your credentials and grant access. Once this has been done, your issue will be resolved.
Note: If you are the Admin of your organisation’s Google Apps account, please refer to this link for more information – Google Support