Any good idea for avoiding such a events on the Front End server:

Log Name:      Lync Server
Source:        LS InterCluster Routing
Date:          1/12/2014 2:17:49 PM
Event ID:      55005
Task Category: (1063)
Level:         Error
Keywords:      Classic
User:          N/A
Computer:      FrontEnd01.internal.fqdn
Description:
Unexpected exception occurred in the InterCluster Routing Application.
 
Exception: Microsoft.Rtc.Sip.UnauthorizedException: Application not authorized. ---> System.Management.ManagementException: Invalid class  
   at System.Management.ManagementException.ThrowWithExtendedInfo(ManagementStatus errorCode)
   at System.Management.ManagementObjectCollection.ManagementObjectEnumerator.MoveNext()
   at Microsoft.Rtc.Sip.ServerAgent.GetWinsipAccountName()
   at Microsoft.Rtc.Sip.ServerAgent.Initialize(ApplicationManifest manifest, Boolean scriptOnly)
   --- End of inner exception stack trace ---
   at Microsoft.Rtc.Sip.ServerAgent.Initialize(ApplicationManifest manifest, Boolean scriptOnly)
   at Microsoft.Rtc.ApplicationService.ApplicationService.Start(EventWaitHandle shutdownEvent, RoleName roleName)

This (and around 10 other similar from other components) appears into Lync log when I try to start the Front End service. So it is not only internal router which is reporting troubles but others like "Intelligent IM Filter Application", "Client Version Filter application", etc..

I tried to re-install the latest updates, but that did not help. The strange thing is, this problem started when I restarted that problematic servers. The pool's other servers have no troubles a

The term "Invalid Class" actually was really a WMI class. When I try to open WMI properties I received an error "invalid Class" and also from the application log I found the following error:

Log Name:      Application
Source:        Microsoft-Windows-WMI
Event ID:      10
Level:         Error
Computer:      FrontEnd01.internal.fqdn
Description:
Event filter with query "select * from MSFT_SCMEventLogEvent" could not be reactivated in namespace "//./root/CIMV2" because of error 0x80041010. Events cannot be delivered through this filter until the problem is corrected.

This I fixed by these:

At first check the WMI status:

c:\>winmgmt /verifyrepository
WMI repository is INCONSISTENT

Then try to fix it:
c:\>winmgmt /salvagerepository
WMI repository salvage failed
Error code:     0x8007041B
Facility:       Win32
Description:    A stop control has been sent to a service that other running services are dependent on.

Failed because there were still some WMI depended services running, stopped them and run again:

c:\>winmgmt /salvagerepository
WMI repository has been salvaged

Now I get

c:\>winmgmt /verifyrepository
WMI repository is CONSISTENT

Reboot and Front End was able to start!