[NiteWave]

please paste the error log relating to 503.

here's an example:

2012-03-07 02:44:28.157 [NOTICE] [85.113.57.238:42742-0#domain.com] Redirect: #1, URL: /index.php
2012-03-07 03:41:47.396 [INFO] [119.63.196.62:2408-0#domain.com] connection to [/tmp/lshttpd/lsphp5.sock.226] on request #0, confirmed, 0, associated process: -1, running: 0, error: Connection reset by peer!
2012-03-07 03:41:47.410 [INFO] [119.63.196.62:2408-0#domain.com] connection to [/tmp/lshttpd/lsphp5.sock.226] on request #0, confirmed, 0, associated process: -1, running: 0, error: Connection reset by peer!
2012-03-07 03:41:47.422 [INFO] [119.63.196.62:2408-0#domain.com] connection to [/tmp/lshttpd/lsphp5.sock.226] on request #0, confirmed, 0, associated process: -1, running: 0, error: Connection reset by peer!
2012-03-07 03:41:47.422 [NOTICE] [119.63.196.62:2408-0#domain.com] Max retries has been reached, 503!
2012-03-07 03:41:47.422 [NOTICE] [119.63.196.62:2408-0#domain.com] oops! 503 Service Unavailable^M
2012-03-07 03:41:47.422 [NOTICE] [119.63.196.62:2408-0#domain.com] Content len: 0, Request line: 'GET /background HTTP/1.1'
2012-03-07 03:41:47.422 [NOTICE] [119.63.196.62:2408-0#domain.com] Redirect: #1, URL: /index.php
2012-03-07 04:59:58.562 [INFO] [157.55.17.88:63532-0#domain.com] connection to [/tmp/lshttpd/lsphp5.sock.279] on request #0, confirmed, 0, associated process: -1, running: 0, error: Connection reset by peer!

above is a complete error log for 503 error happening at 03:41:47, which extracted from error.log.

please check if your recent 503 error has same pattern as above: before "Max retries has been reached, 503!", there is a line like
"connection to [/tmp/lshttpd/lsphp5.sock.226] on request #0, confirmed, 0, associated process: -1, running: 0, error: Connection reset by peer!"

above log is extracted from a server which just upgraded to 4.1.11 recently, and all are same pattern(on request #0, confirmed, 0, associated process: -1).