It means that your server couldn't reach any of our license servers, probably due to a networking issue for its periodic floating license management.
The emails are sent a little bit too quickly in CBSubs 1.0.3, but it will continue working under normal circumstances (e.g. if you are not changing server or server address same time), so if you don't get multiple of those during more than a day in a row, there is nothing to worry about.
We will improve that in next CBSubs release.
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To upgrade your license with more users, just go to your profile here on joomlapolis, and inside your existing license, click "Change Options" button, then choose your new option(s) and follow directions to checkout. At this time, only the price difference is due. Nothing special to do on your site(s), their license will be automatically upgraded.