Altiris agent don't send reboot event to NS

We run NS 7.5.1670 and agent is installed on 2008R2 sp1

Facing an issue with patch compliancy report, some servers show reboot required but in fact they don't.

Looking to the altiris agent logs i've found entries saying agent can't send reboot event:

<event date='11/11/2014 21:02:27.8100000 +01:00' severity='4' hostName='BPVWMTS01' source='SoftwareUpdateAgent' module='PatchMgmtAgents.dll' process='AeXNSAgent.exe' pid='1828' thread='1528' tickCount='190742'>
  <![CDATA[rebootCheck() - Reboot detected - sending reboot event to NS server]]>
</event>
<event date='11/11/2014 21:02:27.8730000 +01:00' severity='4' hostName='BPVWMTS01' source='SMFAgent_Agent' module='smfagent.dll' process='AeXNSAgent.exe' pid='1828' thread='372' tickCount='190804'>
  <![CDATA[SMF thread continue entering operation mode]]>
</event>
<event date='11/11/2014 21:02:27.9970000 +01:00' severity='2' hostName='BPVWMTS01' source='CoEventTransport::raw_SendQueued' module='AeXNSAgent.exe' process='AeXNSAgent.exe' pid='1828' thread='1528' tickCount='190929'>
  <![CDATA[Error copying event to queue: The process cannot access the file because it is being used by another process. (0x80070020)]]>
</event>
<event date='11/11/2014 21:02:28.0130000 +01:00' severity='2' hostName='BPVWMTS01' source='AeXNSEvent::raw_SendQueued' module='AeXNSEvent.dll' process='AeXNSAgent.exe' pid='1828' thread='1528' tickCount='190945'>
  <![CDATA[The process cannot access the file because it is being used by another process. (0x80070020)]]>
</event>
<event date='11/11/2014 21:02:28.0130000 +01:00' severity='2' hostName='BPVWMTS01' source='PatchMgmtSoftwareUpdateAgentConfiguration' module='PatchMgmtAgents.dll' process='AeXNSAgent.exe' pid='1828' thread='1528' tickCount='190960'>
  <![CDATA[Failed to send Reboot NSE - The process cannot access the file because it is being used by another process. (0x0-2147024864)]]>
</event>

What may cause this behavior?

Thank you all in advance!