Account Keepgetting Lockout from TMG Server

Hi All,

one domain user account keep getting lockout every 30min from our TMG server. i have restarted the TMG server many times. earlier i was having only TMG SP1. after i encounter this lockout problem i have upgraded my TMG server to SP2.

my currently TMG Version : 7.0.9193.500

i have captured below event error from TMG server ( Event ID : 4648, Source : Microsoft Windows security )

A logon was attempted using explicit credentials.

Subject:
    Security ID:        NETWORK SERVICE
    Account Name:        TMGServerName$
    Account Domain:        WORKGROUP
    Logon ID:        0x3e4
    Logon GUID:        {00000000-0000-0000-0000-000000000000}

Account Whose Credentials Were Used:
    Account Name:        userid
    Account Domain:        domain
    Logon GUID:        {00000000-0000-0000-0000-000000000000}

Target Server:
    Target Server Name:    Domain.FQDN.org
    Additional Information:    Domain.FQDN.org

Process Information:
    Process ID:        0x2f98
    Process Name:        C:\Program Files\Microsoft Forefront Threat Management Gateway\wspsrv.exe

Network Information:
    Network Address:    -
    Port:            -

This event is generated when a process attempts to log on an account by explicitly specifying that account’s credentials.  This most commonly occurs in batch-type configurations such as scheduled tasks, or when using the RUNAS command.

i am not sure, why the account keep getting lockout from TMG, any suggestion would really helpful.

srini