For those still encountering this issue:

AWS offers an instance type called "Burstable Performance Instances," which is likely the type you selected for your development server (instances starting with "t"). The key concept behind these instance types is CPU Credits. Essentially, there is a threshold for CPU usage, and if you use more than this threshold, credits are deducted. Conversely, if your CPU usage remains below the threshold, credits are accumulated over time.

The problem arises when all your CPU credits are consumed. When this happens, your server experiences significant performance throttling, which can even render it unable to accept SSH connections. In this case, it is not the firewall or security system blocking your access—it’s simply that your instance is so severely throttled that it cannot handle any additional load.

In most cases, as your CPU usage drops, new CPU credits will be earned, and the server's performance will eventually return to normal. At this point, you should regain the ability to connect via SSH.