We need more information to figure out what exactly causing the problem.
Please try turn on LSWS debug logging by changing debug level to "high", post the section of error.log for the failed request.
Strace the ruby process. First send a good request to let the ruby process start, then do "strace -p <pid_of_ruby>", trace the process with larger PID. send a request result in 503 error, post the strace result.
check rails' log files.