A server is full when there are no more CPU/GPU/RAM resources on that server. Each SkyComputer occupies a chunk of the server's resources as it runs. When you receive a "Full" error, that means that the entire server cluster which contains your SkyComputer’s SkyStorage is currently hosting its max possible amount of concurrent users. SkyComputers move from server cluster to server cluster based on complex load balancing algorithms. At a time when the server is busy due to high user load, many clusters are maxed out, preventing further users from connecting.
Waiting couple of minutes and trying again should help. Choosing the Gamer Performance package should also increase your chances of accessing your SkyComputer as there are more Gamer slots available than Pro on each server.
We are working on finding a good solution, so you don't have to see this message very often in the future.