There are two cases in which you'll need to re-authenticate your Gmail/G Suite account on ScopeLeads:
1: Getting a popup that prompts you to do so
This can happen due to various reasons, either us updating our Google API so you need to log in on the newest version, your authentication being a few months old so you need to log back in again or some other factors that we can't fully control.
2: Getting 'Error' messages on attempts to send
This is directly connected to case 1, whereas if you try to send with a broken login token you'll end up getting an error.
Special case would be you not being logged in at all, so there's no Gmail/Gsuite account to send with.
A simple re-authentication will do the trick.
Navigate to your profile page and click on 'Disconnect gmail' link on your email settings:
After that a red G button will show up, simply click on it and go through the login process:
After that click save on the bottom of the page and problem solved!