That's an anti-spoofing error, which means that the firewall is seeing traffic incoming on an interface, where IP packets with such a source address are not expected.
You have to check the anti-spoofing settings of all interfaces of that firewall (in Smart Console -> Gateway Settings -> Network Management -> Edit interfaces -> Topology).
Please take in mind, that having an interface with anti-spoofing set to "Internet (External)" does not mean it will accept all IP sources. Instead it means it will accept all IP sources which are not covered by the specific anti-spoofing settings of all other interfaces.
Maybe you use groups on some internal interfaces for anti-spoofing and someone added a network to that group, not knowing it will affect anti-spoofing?
Maybe you use anti-spoofing defined by routes and (dynamic) routing has changed?