Re: “Stack overflow at line 15” and site not accepting new users
As Paul said, you need to distill your environment down to the least common denominator. Switch to the default BP theme and make sure you do not have any other plugins besides BP activated. If the problem goes away, then it has something to do with your theme, one of your installed plugins, or a combination of both.
The next step, then, is to figure out what is causing the problem. So, first switch back to your custom theme. If your site still runs without issue, then it is one or more of your plugins. If the site breaks, then it is your theme. If the theme seems fine, then start reactivating one plugin at a time until the problem returns.
Also, you should look in your server’s log files to see if there are any clues as to what might be going on.