I have a Multi-SAN certificate issued by Godaddy which has four subject alternate names.
to use this in an Exchange environment, I have added the external hostname and internal hostnames of my exchange box as SANs - so both internal and external OWA users can access the server without certificate errors.
I have now added a Lync server to the environment, and popped the same certificate on that machine, hoping to use one of the other SAN names to provide that service.
Lync users can connect fine, and all seems well - but OWA integration just fails outright, with the "Instant Messaging is not available right now" error message.
I suspect that the fact that the same certificate is on both the OWA box, and the Lync box (albeit referencing different SANs of the same cert) is the problem.