Or, let us help, reach out to SaneBox support.

IMAP client reporting "error authenticating with Gmail: Web login required."

Occasionally, Gmail requires you to log in on their website in order to continue using IMAP clients.  

Simply go to Gmail’s Sign In page, log out if you are currently logged in, and then log in to your account.  It should fix itself.  

Please open a Support Request if this does not work for you.