ID=15 is a common error when trying to play a Graphictoria game. It usually occurs if the host did not port forward, the internet speed is too slow, or the host of the server port forwarded incorrectly.
A thread on the forums created by Whimee, explains this in detail. The thread states,
"First of all, the ID=15 error means that whatever server you joined was incorrectly hosted, meaning the hoster didn't port forward or input the right things. It's not your fault you are getting this error, so join another server!
Failed to connect to server is also not your fault, as it is a result of a game not even being attempted to be hosted by the hoster. Port forwarding is something we cannot help you with, as it requires work from yourself. Look up tutorials online and ask other members!"