I have seen this issue in the past and it can be something with the boot media or the tasks that are present in the system even though all is working fine but with that message "Check your internet connection or change DNS or mirror server." I have seen that before. If removing the tasks and then trying to update the Antivirus and or the update to the latest version you might want to submit a support ticket so we can look into it deeper. I have also seen a re-installation or new boot media has worked as well.