I have a Lighttpd server plus mod_fastcgi. And today I had Internal Server Error 500. I've checked my error log and it goes like this:

2010-04-22 22:59:14: (server.c.1464) server stopped by UID = 0 PID = 3332
2010-04-22 22:59:15: (mod_fastcgi.c.1768) connect failed: No such file or directory on unix:/tmp/php.socket-5
2010-04-22 22:59:15: (mod_fastcgi.c.2956) backend died; we'll disable it for 5 seconds and send the request to another backend instead: reconnects: 0 load: 1
2010-04-22 22:59:15: (mod_fastcgi.c.2709) child died somehow, waitpid failed: 10
2010-04-22 22:59:15: (server.c.1464) server stopped by UID = 0 PID = 3332
2010-04-22 22:59:15: (server.c.1464) server stopped by UID = 48 PID = 1385
2010-04-22 22:59:15: (server.c.1464) server stopped by UID = 48 PID = 1385
2010-04-22 22:59:15: (server.c.1464) server stopped by UID = 48 PID = 1385
2010-04-22 22:59:15: (server.c.1464) server stopped by UID = 48 PID = 1385

What to do to find out what can be cause of this?