I need to allow a incoming subnet, so tried to setup a filter exception:
Source Interface: WAN-IFC,
Source Address Type: Network,
Source IP Address:, (this segment is outside from FW!)
Source Subnet Mask:,
clicking next I get a "Runtime error" or a message "Error on this Page".
Screenshot attached.

BM 3.8 sp4 ir3, NW 6.5 SP5.

Any ideas how to create a subnet filter?