MSExchange Management Event log error

We have a single sbs2011 running exchange and every half an hour we get the following 2 errors in our MSExchange Management Event log :-

Error      18/02/2014 14:30:00      MSExchange CmdletLogs      6      General

Cmdlet failed. Cmdlet Get-User, parameters {Identity=NT AUTHORITY\SYSTEM}.

With Details :-

Get-User
   {Identity=NT AUTHORITY\SYSTEM}
   hds1.local/Domain Controllers/SBS2011
   S-1-5-21-177518626-3322050215-1828347314-1014
   S-1-5-21-177518626-3322050215-1828347314-1014
   ServerRemoteHost-Unknown
   6240
   
   13
   00:00:00.0312002
   View Entire Forest: 'False', Default Scope: 'domain.local', Configuration Domain Controller: 'server.domain.local', Preferred Global Catalog: 'server.domain.local', Preferred Domain Controllers: '{ server.domain.local }'
   Microsoft.Exchange.Configuration.Tasks.ManagementObjectNotFoundException: The operation couldn't be performed because object 'NT AUTHORITY\SYSTEM' couldn't be found on 'server.domain.local'.
   Context
 
   
Followed by :-


Error      18/02/2014 14:30:00      MSExchange CmdletLogs      6      General

Cmdlet failed. Cmdlet Get-User, parameters {Identity=NT AUTHORITY\SYSTEM}.

With details :-

Get-User
   {Identity=NT AUTHORITY\SYSTEM}
   hds1.local/Domain Controllers/SBS2011
   S-1-5-21-177518626-3322050215-1828347314-1014
   S-1-5-21-177518626-3322050215-1828347314-1014
   ServerRemoteHost-Unknown
   6240
   
   13
   00:00:00.0780005
   View Entire Forest: 'False', Default Scope: 'domain.local', Configuration Domain Controller: 'server.domain.local', Preferred Global Catalog: 'derver.domain.local', Preferred Domain Controllers: '{ server.domain.local }'
   Microsoft.Exchange.Configuration.Tasks.ManagementObjectNotFoundException: The operation couldn't be performed because object 'NT AUTHORITY\SYSTEM' couldn't be found on 'server.domain.local'.
   Context


Can anyone advise on what this indicates, and what we need to do to resolve it??

Any help appreciated.

Many thanks.