Temporary "File not found" error

About one month ago I had set up a new server configuration, since then
struggle with an annoying temporary error.
Most of the time everything works fine, but after some period of time, I
sometimes receive “File not found” errors when I try to acces a PHP

access log:
a.b.c.d - - [20/Jan/2013:13:14:38 +0100] “GET /intern/phpMyAdmin/
404 47 “-” “Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:18.0)
Firefox/18.0” “-”
a.b.c.d - - [20/Jan/2013:13:14:39 +0100] “GET /intern/phpMyAdmin/
200 2378 “-” “Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:18.0)
Gecko/20100101 Firefox/18.0” “-”

error log:
2013/01/20 13:14:38 [error] 26428#0: *6892 FastCGI sent in stderr:
script unknown” while reading response header from upstream, client:
a.b.c.d, server: abc.de, request: “GET /intern/phpMyAdmin/ HTTP/1.1”,
upstream: “fastcgi://”, host: “abc.de

Two request, the first on throws an error, the second one is fine.
So there is only one of the two php-fpm workers affected?
Since it is working most of the time, I don’t think it has something to
with file permissions?

Perhaps someone has an idea whats wrong with my configuration?

Thanks in advance!

nginx config:
root /srv/www/abc.de/public/www;
access_log /srv/www/abc.de/log/access.log main;
error_log /srv/www/abc.de/log/error.log warn;

location / {
index index.html index.htm index.php;

location ~ .php$ {
if ( !-f $request_filename ) {
return 404;

    include         /etc/nginx/fastcgi_params;
    fastcgi_split_path_info ^((?U).+\.php)(/?.+)$;

    fastcgi_param   DOCUMENT_ROOT           /public/www;
    fastcgi_param   SCRIPT_FILENAME

fastcgi_param PATH_INFO $fastcgi_path_info;
fastcgi_param PATH_TRANSLATED
fastcgi_index index.php;

fastcgi_param QUERY_STRING $query_string;
fastcgi_param REQUEST_METHOD $request_method;
fastcgi_param CONTENT_TYPE $content_type;
fastcgi_param CONTENT_LENGTH $content_length;

fastcgi_param SCRIPT_FILENAME $request_filename;
fastcgi_param SCRIPT_NAME $fastcgi_script_name;
fastcgi_param REQUEST_URI $request_uri;
fastcgi_param DOCUMENT_URI $document_uri;
fastcgi_param DOCUMENT_ROOT $document_root;
fastcgi_param SERVER_PROTOCOL $server_protocol;

fastcgi_param GATEWAY_INTERFACE CGI/1.1;
fastcgi_param SERVER_SOFTWARE nginx/$nginx_version;

fastcgi_param REMOTE_ADDR $remote_addr;
fastcgi_param REMOTE_PORT $remote_port;
fastcgi_param SERVER_ADDR $server_addr;
fastcgi_param SERVER_PORT $server_port;
fastcgi_param SERVER_NAME $server_name;

fastcgi_param HTTPS $https;

PHP only, required if PHP was built with --enable-force-cgi-redirect

fastcgi_param REDIRECT_STATUS 200;

fastcgi_connect_timeout 120;
fastcgi_send_timeout 180;
fastcgi_read_timeout 180;
fastcgi_buffer_size 128k;
fastcgi_buffers 4 256k;
fastcgi_busy_buffers_size 256k;
fastcgi_temp_file_write_size 256k;
fastcgi_intercept_errors on;

php-fpm pool config:
prefix = /srv/www/abc.de/

listen =
listen.backlog = -1
listen.allowed_clients =
listen.owner = www-data
listen.group = www-data
listen.mode = 666

user = www-data
group = www-data

pm = dynamic
pm.max_children = 30
pm.start_servers = 2
pm.min_spare_servers = 1
pm.max_spare_servers = 10
pm.max_requests = 500

pm.status_path = /php_pool_abc_status
ping.path = /abc_ping
ping.response = abc_pong

request_terminate_timeout = 0
request_slowlog_timeout = 1
slowlog = log/php-slow.log
access.log = /srv/www/abc.de/log/abc_pool.access.log

chroot = $prefix
chdir = /public/

security.limit_extensions = .php .php3 .php4 .php5

; env[PATH] = /usr/local/bin:/usr/bin:/bin
env[TMP] = /tmp
env[TMPDIR] = /tmp
env[TEMP] = /tmp

php_flag[display_errors] = off
php_admin_value[error_log] = /log/php_err.log
php_admin_flag[log_errors] = on
php_flag[suhosin.session.encrypt] = off

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