Default Threat Protection is a feature of WatchGuard XTM appliances to automatically block the source of Denial of Service attacks, port scans and other attacks for a certain period of time. However, you should define an exception for your internal servers to prevent them from being blocked.
Using Fireware Policy Manager, navigate to Setup>> Default Threat Protection>> Blocked Sites. Click the Blocked Sites Exceptions tab. Add your internal server IP addresses or the entire subnet. Save the configuration to the appliance.