lighttpd crash with "backend is overloaded"

lighttpd crash with 500 internal error.
connection is about 1000. the load average is only 0.78.
Ram: 2G,
CPU:Intel(R) Pentium(R) 4 CPU 3.20GHz
Lighttpd 1.4.18

Here is the log

Here is config file

 

Hi. Saw you on webhostingtalk. I have the exact same problem. I tried spawn-fcgi and still the same thing. I also tried using nginx and still the same error. I now use monit to restart spawn-fcgi when down. I check every 1second. So the greatest downtime I get is 2 seconds! But it happens all the time still.