The error indicates an issue during Phase 1 of the IKE handshake, which is used for establishing secure key exchange in IPSec VPNs. If the Diffie-Hellman group is legacy (e.g., Group 1 or 2), it might no longer be supported by modern systems, causing the connection to fail. Updating to a stronger Diffie-Hellman group (e. g., Group 14 or 19) resolves this issue. This aligns with CASP+ objectives related to secure communications and cryptographic protocols (3.2).