This has been fixed. I assume many of you will see this soon using authorize.net sandbox account. This is a generic error that could be anything, but in my case it was that fact that Kentico was using TLS version 1.1. A few days ago, authorize.net banned the use of TLS 1.1, so this error was authorize.net refusing the connection.
This accouncement comes from here, and Kentico will be affected for those of you who use .NET 4.5.2 or .NET 4.5
In order to solve this, please follow these instructions and make sure to only allow TLS 1.2