I returned from holidays recently to find my Skype for Business client reporting the familiar “Limited External Calling” error.

A quick RDP to my Lab’s Edge server revealed stopped services and this error in the Event Log:

Event Properties - Event 19005, LS Audio/Video Authentication Server

LS Audio/Video Authentication service could not be started.

Exception: System.IO.FileNotFoundException: Could not load file or assembly 'Microsoft.Rtc.Collaboration, Version=6.0.0.0, Culture= neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

Filtering the Event Log by this error I saw it pops up from time to time in brief spurts, then vanishes as quickly as it arrived. The date-stamp of its first recent occurrence looked interesting – that turned out to be the date I applied CU6 to the server. I’m guessing I was distracted or maybe rebooted it “into service” (ha!) and moved on to something else without checking it properly.

The fix was to run bootstrapper and it came good straight away.

PS C:\Program Files\Skype for Business Server 2015\Deployment> .\Bootstrapper.exe
PS C:\Program Files\Skype for Business Server 2015\Deployment> Start-CsWindowsService

– G.