502 Bad Gateway

Hi, I have a FortiGate 600c (v5.0,build0252) with navigation via webproxy. A web site reply always " 502 Bad Gateway" but the site exist and work. http://pros.lib.unimi.it/login?url=http://link.springer.com/journal/volumesAndIssues/15010 --> not work (502 Bad Gateway), but with another internet line work But if I check the link http://link.springer.com/journal/volumesAndIssues/15010 it work. Any idea? Jonathan