zcs 6.0.5 system failure after upgrade

[KennyDeb]

zmcontrol -v
Release 6.0.5_GA_2213.UBUNTU6 UBUNTU6 FOSS edition.

After upgrade from 5.0.20 to 6.0.5 i found this error

Feb 16 09:25:59 mail postfix/smtpd[3880]: connect from unknown[172.16.2.92]
Feb 16 09:25:59 mail saslauthd[4695]: zmauth: authenticating against elected url 'https://mail.siamraj.com:7071/service/admin/soap/' ...
Feb 16 09:25:59 mail saslauthd[4695]: zmpost: url='https://mail.siamraj.com:7071/service/admin/soap/' returned buffer->data='<soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope"><soap:Header><context xmlns="urn:zimbra"/></soap:Header><soap:Body><soap:Fault><soap:Code><soa p:Value>soap:Receiver</soap:Value></soap:Code><soap:Reason><soap:Text>system failure: ZimbraLdapContext</soap:Text></soap:Reason><soapetail><Error xmlns="urn:zimbra"><Code>service.FAILURE</Code><Trace>btpool0-9://mail.siamraj.com:7071/service/admin/soap/:1266287159662:2474ea7d0fecb2c4</Trace></Error></soapetail></soap:Fault></soap:Body></soap:Envelope>', hti->error=''
Feb 16 09:25:59 mail saslauthd[4695]: auth_zimbra: surasak@siamraj.com auth failed: system failure: ZimbraLdapContext
Feb 16 09:25:59 mail saslauthd[4695]: do_auth : auth failure: [user=surasak@siamraj.com] [service=smtp] [realm=siamraj.com] [mech=zimbra] [reason=Unknown]
Feb 16 09:25:59 mail postfix/smtpd[3880]: warning: unknown[172.16.2.92]: SASL LOGIN authentication failed: authentication failure
Feb 16 09:25:59 mail postfix/smtpd[3880]: lost connection after AUTH from unknown[172.16.2.92]
Feb 16 09:25:59 mail postfix/smtpd[3880]: disconnect from unknown[172.16.2.92]

after i restart by /etc/init.d/zimbra restart it can use in normal but still happen in next day or tree day.

[KennyDeb]

Any help? it still happen every morning time T_T