It seems that Check Point has changed the way that policies are compiled/handled. At one installation I saw a VPN connection break right after the policy was installed the first time from the upgraded management server. Right after that “According to the policy the packet should not have been decrypted” started appearing in the logs and traffic started being dropped.
After a bit of looking around on the Check Point support site about the message it pointed to multiple objects having similar VPN domains. After going through the object database, I managed to find a “copy” of the peer GW object. Basically 2 gateway objects had same external IP address and similar VPN domains defined. Fortunately that “duplicate” entry wasn’t in use so I deleted it. After deleting the duplicate entry and pushing the policy again, the traffic started flowing again.
I wonder why the upgrade verification process showed no errors or warnings. To me it seems they should have shown some sort of warnings about it, as it breaks things.