Outlook 2010 with Exchange 2003 account freezes with event ID 515 in security event log

My Outlook 2010 freezes a couple of times per day. I am connected to a Exchange 2003 server. Sometimes, when I try to send an email or open an appointment, the interface completely freezes. After a few seconds a balloon pops up saying:Outlook is contacting Server.network.local (where 'Server' is the primary domain controller and not the Exchange server).
I checked the event log of that server and I found out that every time my Outlook freezes, three notices are written to the Security Event log:

Event Type: Success Audit Event
Source: Security Event
Category: System Event Event ID: 515
Date: 4/8/2011 Time: 4:45:26 PM
User: NT AUTHORITY\SYSTEM
Computer: Server Description: A
trusted logon process has registered
with the Local Security Authority.
This logon process will be trusted to
submit logon requests.

Logon Process Name: KSecDD

For more information, see Help and
Support Center at
http://go.microsoft.com/fwlink/events.asp.

Event Type: Success Audit Event
Source: Security Event
Category: System Event Event ID: 515
Date: 4/8/2011 Time: 4:45:25 PM
User: NT AUTHORITY\SYSTEM
Computer: Server Description: A
trusted logon process has registered
with the Local Security Authority.
This logon process will be trusted to
submit logon requests.

Logon Process Name: KSecDD

For more information, see Help and
Support Center at
http://go.microsoft.com/fwlink/events.asp.

Event Type: Success Audit Event
Source: Security Event
Category: System Event Event ID: 515
Date: 4/8/2011 Time: 4:45:25 PM
User: NT AUTHORITY\SYSTEM
Computer: Server Description: A
trusted logon process has registered
with the Local Security Authority.
This logon process will be trusted to
submit logon requests.

Logon Process Name: Winlogon\MSGina

For more information, see Help and
Support Center at
http://go.microsoft.com/fwlink/events.asp.

Who can help me solving the freezing Outlook?