Some IPsec packets are dropped unexpectedly on a computer that is running Windows Server 2008 or Windows Vista
Some IPsec packets on a computer that is running Windows Server 2008 or Windows Vista are dropped unexpectedly. Additionally, applications or services which relay on IPsec communication may encounter failures.
Note: This issue occurs when the Group Policy is changed on the computer and when the network load is high.
Additionally, when this issue occurs, the following event is logged in the security event log:
Log Name: Security
Event ID: 4963
Task Category: IPSec Driver
Keywords: Audit Failure
IPSec dropped an inbound packet that should have been secured. This is usually due to the remote computer changing its IPSec policy without informing this computer. This could also be a spoofing attack attempt.
For more information on this issue, including potential causes, workarounds, and resolutions, see: Microsoft KB Article KB980864.