Now i am not sure which steps you already have taken or what Check Point version you are running but
there are some things i would try in an effort to rule out some issues.
IKE Version, are running v1 or v2 in the community ? Possible to switch and test ?
IPv4 summarization, Check Point fw is going to try to summarize the networks in the encryption domain which will cause issues if the other end has 2 /24's for example and Check Point is presenting a /23.
Are there more VPN tunnels to this Check Point endpoint ? Have you considered trying to use "Encryption Domain per Community"