When you see an SSL certificate verification error while attempting to use a SpiderOak product, the most common reason is that you have an old version with an expired certificate. The solution is to update it to the current version of SpiderOak One Backup, Groups Backup, Semaphor, or Encryptr. Updates are always free and will not affect your plan.
If you see this error even after upgrading to the latest version, it is because the server answering the request and claiming to be us is not an authentic SpiderOak server. Many paid or otherwise restricted Wi-Fi hotspots do this as part of their measures to ensure that all users are paid and authorized, and in this case the only solution is to use a different means of connection to the Internet.
Less benignly, this could indicate that an attacker, often a government authority, is performing a man-in-the-middle SSL attack on your connection.