The issue issue is documented in Cisco bug ID CSCei50549.
This happens when a PIX Firewall is upgraded from version 6.x (four-tuple key version) to version 7.x (five-tuple key version) and is used in failover mode with another similar PIX device.
For a workaround, obtain a new five-tuple key from Cisco Technical Support.
As an alternative, upgrade to software version7.0(2). Refer to Software Download: Cisco PIX Security Appliance Software
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