File upload with lighttpd

I’m running a rails app with lighttpd 1.4.18 and fcgi.
Whenever I try to upload a file larger then 1Mb or so I get a “503 -
Service Not Available” error.
Any clue?

And this is what the server log has to say:

2008-01-14 02:43:24: (network_writev.c.249) mmap failed: Invalid
argument /var/tmp/lighttpd-upload-ZcEVWV 8
2008-01-14 02:43:24: (mod_fastcgi.c.2996) write failed: No such file or
directory 2