! symbol and "Limited External Calling" error in OCS this morning

Hmm, not sure if that error is a red herring or not.

Doing a little more digging, Ive found an error in the Office Communications Server log 

Category: 1043
Event ID: 19007

A/V Authentication Certificate used for generating credentials not found in the machine store or it may be invalid. A/V Authentication Edge Service will stop.

Certificate Serial Number: '06DC814EF1444848B3E576073AC88689' Issuer 'CN=DigiCert High Assurance CA-3, OU=www.digicert.com, O=DigiCert Inc, C=US'
Cause: A/V authentication certificate was not found in local machine store or it is invalid
Resolution:
Install the certificate into local machine store and check if the config parameter has the correct serial number. If the certificate was found, check if it is valid.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

Which is then followed by an error that says:

Category: 1043
Event ID: 19005

A/V Authentication Edge Service could not be started.

Exception: Microsoft.Rtc.MRAS.MRASException: server sharedsecret certificate not found in the local machinestore
   at Microsoft.Rtc.MRAS.Crypto.GetValidCertificate(String issuerName, String serialNumber, Boolean isBankCert)
   at Microsoft.Rtc.MRAS.Crypto..ctor(Byte[] bankCertSN, Byte[] bankCertIssuer)
   at Microsoft.Rtc.MRAS.CredentialsGenerator.InitializeCrypto()
   at Microsoft.Rtc.MRAS.CredentialsGenerator..ctor(Configuration config)
   at Microsoft.Rtc.MRAS.Core.Initialize()
   at Microsoft.Rtc.MRAS.Core..ctor(ServiceStopHandler serviceStop)
   at Microsoft.Rtc.MRAS.Server.OnStart()
Cause: Internal error.
Resolution:
Examine the details in the associated event log entry to determine the potential cause and report to Product Support Services.

For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.

All of which would seem to indicate the server either cant find the DigiCert UC certificate on this box (which wasnt due to expire until 2/2011), or it cant use it anymore.

However when I look at the cert chain for the UC cert on the box, everything looks fine (see attachment)

(And yes, I checked all of the certs above this in the chain, and they all check-out.)

Now I grant you that Ive never installed a UC cert myself, but I dont see any problems here that would explain our issue.

At present Im just poking-around, trying to find anything on Google that might shed some light on all of this, so if anyone has a brilliant idea, Id be very glad to hear it.