After logging in to the OCS server and adding the Office Communications Server 2007 MMC snap-in to the console I noticed the certificate used was expired at 4/4/2013, which was yesterday. This is visible in Standard Editions(depending on which edition used ofcourse) > <server-name> > <server-fqdn pool> > General settings.
The certificate is used to encrypt SIP communications over TLS, so all clients use it to communicate with the OCS server.
To solve it a new certificate needs to be requested, this can be done by Right clicking the server pool > Certificates > Create a new certificate. The rest off the wizard depens on how your PKI is configured or company procedure. I choose Send the request immediately to an online certification authority > (Ofcourse information in the images is environment specific)
Create a request to a certifate authority, probably you internal CA and issue. Afterwards you need
to run the wizard again and import the certificate pending request.
For more information about OCS and Certificates:
For more information about PKI and CA: