Web page blocked with error 'Web filter Service error N/A'

Hello I had a problem with Web filter Service where all users could not browse because the fortinet block page was displayed. With the message 'Web filter Service error N/A' I checked the security profile that have the browsing policies and this has enabled the option “Allow websites when a rating error occurs” which in several forums indicate that it is a workaround for scenarios like this at the level of traffic captures the action was Passthrough. Can anyone help me understand what could have happened or what revisions I can make to understand. Thank you very much