404 error while loading page, but working fine

FancyBox is working fine, but testing my site using WebPageTest.org I found the following error when a page is loaded:

URL: http://[mywebsite]/wp-content/plugins/fancybox-for-wordpress/fancybox/null
Host: [mywebsite]
IP: xxxxxxxxx
Location: Netherlands
Error/Status Code: 404
Start Offset: 11.208 s
Time to First Byte: 983 ms
Content Download: 0 ms
Bytes In (downloaded): 7.7 KB
Bytes Out (uploaded): 0.6 KB

REQUEST:
GET /wp-content/plugins/fancybox-for-wordpress/fancybox/null HTTP/1.1
Accept: */*
Referer: http://[mywebsite]/
Accept-Language: en-US
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C; PTST 2.350)
Accept-Encoding: gzip, deflate
Host: http://www.drentsheerlijk.nl
Connection: Keep-Alive
Cookie: bb2_screener_=1363252877+72.66.115.14; __utma=121136733.1067623067.1363252875.1363252875.1363252875.1; __utmb=121136733.1.10.1363252875; __utmc=121136733; __utmz=121136733.1363252875.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none)

RESPONSE:
HTTP/1.1 404 Not Found
Date: Thu, 14 Mar 2013 09:21:19 GMT
Server: Apache/1.3.41 (Unix) mod_gzip/1.3.26.1a PHP/4.4.9 mod_throttle/3.1.2 mod_psoft_traffic/0.2 mod_ssl/2.8.31 OpenSSL/0.9.8b
Cache-Control: no-cache, must-revalidate, max-age=0
Content-Encoding: gzip
Expires: Wed, 11 Jan 1984 05:00:00 GMT
Pragma: no-cache
Vary: Accept-Encoding,*
X-Pingback: http://www.drentsheerlijk.nl/xmlrpc.php
X-Powered-By: PHP/5.2.9
Set-Cookie: bb2_screener_=1363252879+72.66.115.14; path=/
Keep-Alive: timeout=10, max=87
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8

http://ww.wp.xz.cn/extend/plugins/fancybox-for-wordpress/