Sorry, the webpage I´m trying to access is:
The error you are receiving suggests that something within the header of the site is violating the HTTP application defense in place.
Can you tell us which version you are running as between versions 6, 7 & 8 the way to approach this is quite different.
I´m using 8.2.1 version
In v8, what you can do is to go to the Application Defenses screen (Policy -> Application Defenses -> Defenses), locate the default HTTP defense and make a duplicate copy.
Edit this new entry and you'll probably find that the setting you want to relax is likely to be located under the HTTP URL Control section (but could be in the HTTP Request or Reply tabs). Once you've changed the setting you can then create a new application defense group, with the HTTP option configured to use your new HTTP app defense and, finally, apply this new defense group to your rule.
Alternatively, if you don't want to concern yourself too much with individual settings, you could just straight to the Application Defense Group screen, make a duplicate copy of the group currently being used in the affected rule, set the HTTP option to "None" and apply this to the rule. This will result in the rule performing no HTTP-level inspection at all.
Thanks very much Phil.