The anti-virus engine is experiencing an internal failure trying to scan that resource, and because the anti-virus blade is set to "fail closed" the resulting action is a Prevent. Creating an exception for that resource will not help since it only changes the final decision rendered (Prevent/Detect/Inactive) but does not stop the scanning of that resource and therefore the internal failure that is occurring. It probably has to do with the scanned resource exceeding the fixed size of the SFT buffer on the firewall, please see the following SK for the fix: sk139292: "Failure-reject: unknown error" in Anti-Virus log, traffic fails
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