Weird content blocking by webproxy

Dear,

I have an issue with the webproxy. It's on our wifi guest network. A couple of sites cannot be viewed. This example is for Facebook pictures. I'm still running 8.306 on this machine, so I hope it's not that...
The proxy is transparent for this network.

What I see in the /var/logs/http.log file is the following;

2013:01:16-07:55:32 fw01 httpproxy[9920]: id="0062" severity="info" sys="SecureWeb" sub="http" name="web request blocked, forbidden url detected" action="block" method="GET" srcip="172.16.28.233" dstip="" user="" statuscode="403" cached="0" profile="REF_HttProWifigasten (WIFI-Gasten)" filteraction="REF_HttCffWifigasten (WIFI-Gasten)" size="6315" request="0xc3f9b450" url="http://profile.ak.fbcdn.net/hprofile-ak-ash3/c0.66.156.156/s148x148/550743_10150882884332180_1395024970_a.jpg" exceptions="" error=""

2013:01:16-07:55:33 fw01 httpproxy[9920]: id="0001" severity="info" sys="SecureWeb" sub="http" name="http access" action="pass" method="GET" srcip="172.16.28.233" dstip="95.100.96.195" user="" statuscode="200" cached="4" profile="REF_HttProWifigasten (WIFI-Gasten)" filteraction="REF_HttCffWifigasten (WIFI-Gasten)" size="7821" request="0xaa0d3198" url="http://profile.ak.fbcdn.net/hprofile-ak-prn1/c12.12.156.156/s148x148/65124_463157000387000_1136492584_a.jpg" exceptions="" error="" category="177,195" reputation="unverified" categoryname="Content Server,Social Networking" content-type="image/jpeg" application="facebook"

The first URL is being blocked, but I don't understand why... Forbidden URL detected...? To me that doesn't make sense because FB isn't blocked.

Can somebody help me out here?
Thank you very much!
With kind regards,
Erwin ter Hofstede