This happens because the Data Encryption Standard (DES)/Secure Hash Algorithm (SHA) is not supported by the VPN client.
In order to resolve this issue, try this workaround:
Configure the encryption to 3DES instead of DES with SHA. If the PIX Firewall is not configured for 3DES, try DES with Message Digest 5 (MD5).
Refer to these documents for more information:
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