event id 521 Unable to log events to security

Some users always lock out in active directory
When i looking for the security logs
i getting this message:

Unable to log events to security log:
       Status code:            0xc0000008
       Value of CrashOnAuditFail:      0
       Number of failed audits:      56

Event xmlns="http://schemas.microsoft.com/win/2004/08/events/event">
- <System>
  <Provider Name="Security" />
  <EventID Qualifiers="0">521</EventID>
  <TimeCreated SystemTime="2011-10-04T15:16:24.000000000Z" />
  <Security UserID="S-1-5-18" />
- <EventData>

i cant figure it out what cuse the locking of those users.

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David Johnson, CD, MVPOwnerCommented:
is your security log full?

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SenxAuthor Commented:
Yes Thx
i just figure it out
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Windows Server 2008

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