This error is found in SmartLog: Auth exchange: Sending notification to peer: Authentication failed MyAuthMethod: Pre-shared secret.
These logs are found in the $FWDIR/log/vpnd.elg file:
[ikev2] dbCommunityHandle::getPresharedSecret: No shared secret on the community.
[ikev2] ikeSimpOrder::getPresharedSecret: No shared secret on the community (order 8113931, ref count 1).
[ikev2] sign_with_shared_key: invalid key buffer
[ikev2] ikeAuthExchange_r::validateAuthPayload: Validation of Auth payload failed.
There is a duplicate object configured for the remote peer.