Http error is an internal server error as the message specifies. This means your server cannot handle the load. Have you system administrator or server engineers take a look at it and help them troubleshoot the problem.
Simple minds, Simple thoughts!
I figure if you have the courage to get out of bed in the morning, then how bad can the day be.
Well, it looks like you've managed to break it! This is your objective - well done!
I'm assuming that the app works with more than one user, but not as many as 15 ? (in other words your script is written so that it works with multiple users)
I also assume you're expecting to be able to handle more than 15 users. Before you break the bad news to the server/app people, I should also ask whether you have think time included in the script ? 15 users with no think time is putting a very much greater load on the system than 15 users with think time.
Some other things to consider is how unique do your users need to be or how data dependent are they. Along these lines I would look at what kind of verification points you are using. You may be trying to do something that you should not be able too.