cifs and rpcss logon failures

I recently discovered these two failure entries in my security event log on
my 2K3 file server. I looked through the rest of my log and these seem to be
the only ones. I only looked through failures, successes would take a day
and a half or more to go through. My question is, should I be concerned?
Why would these two events have suddenly happened at 11:55 pm? They appear
to originate from a user/workstation that remains on through the night to
receive reports. Should I worry or are these things that normally run but
for some odd reason happened to fail on this occasion? And what is failure
code 0xB? Thanks for any light you can shed.

here are the events:

Event Type: Failure Audit
Event Source: Security
Event Category: Account Logon
Event ID: 673
Date: 8/15/2006
Time: 11:55:00 PM
User: NT AUTHORITY\SYSTEM
Computer: <server-name>
Description:
Service Ticket Request:
User Name: <username>@<domain-name>.LOCAL
User Domain: <domain-name>.LOCAL
Service Name: cifs/<server-name>
Service ID: -
Ticket Options: 0x40800000
Ticket Encryption Type: -
Client Address: <client IP>
Failure Code: 0xB
Logon GUID: -
Transited Services: -


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


Event Type: Failure Audit
Event Source: Security
Event Category: Account Logon
Event ID: 673
Date: 8/15/2006
Time: 11:55:00 PM
User: NT AUTHORITY\SYSTEM
Computer: <server-name>
Description:
Service Ticket Request:
User Name: <username>@<domain-name>.LOCAL
User Domain: <domain-name>.LOCAL
Service Name: RPCSS/<server-name>
Service ID: -
Ticket Options: 0x40800000
Ticket Encryption Type: -
Client Address: <client IP>
Failure Code: 0xB
Logon GUID: -
Transited Services: -


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

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