Thanks to Yaw Theng and the tech support team at Peplink - they identified (and corrected) the configuration problem.
This level of service and support is extraordinary and much appreciated.
The problem was caused by the internal network configuration combined with a DNS attack on an externally visible IP address which mapped to the MAX Transit router across the SpeedFusion VPN:
This issue is solved now. It is due to a 1-to-1 NAT mapping at the balance 380 side.
There is a 1-to-1 NAT mapping setting at the balance 380 side. By configuring that in balance 380, it will forward all the TCP and UDP ports to 192.168.7.1. The IP address 192.168.7.1 is belonging to LAN interface of the MAX Transit. In that LAN interface, there is a DNS server to serve the LAN PC. Therefore, hackers from the internet can send DNS queries to your network and 192.168.7.1 will answer to those bogus DNS queries. Thus, eating up lot of bandwidth at the SFVPN