I tried to login to OCS Web Access (OCS 2007 R2), but I received the following error :

Cannot sign in because the server is temporarily unavailable.

I logged on to the Web Access server and saw that all services where running. With IIS manager I also noticed that the webservice was running and had the correct certificate. I started the Management console for “Microsof Office Communications Server 2007 R2, Communicator Web Access”. Here the information stated that the service was running. Then I noticed that on the “Status” tab with Connectivity the Certificate experation date was still the value of the old certificate, because we recently updated the certificate. I checked that the correct certificate was attached to the site (Properties -> Connectivity -> Server Certificate) and then I restarted the service through the management console of OCS.

This solved the issue. The Status tab now displayed the correct experation date of the certificate and I was able to log in to Web Access.