No fix, expected behavior
IKEv2 is not supported with firewall policies in Traditional VPN mode.
To resolve this issue, either convert the Traditional VPN policy to Simplified VPN mode, or change the IKE version to IKEv1.
For more information, refer to the "Converting a Traditional Policy to a Community Based Policy" section in the VPN R77 Versions Administration Guide
This solution has been verified for the specific scenario, described by the combination of Product, Version and Symptoms. It may not work in other scenarios.