Microsoft released a bunch of patches this last patch Tuesday, one of which was an update to Lync Server, KB2982385, meant to address a security vulnerability that could allow a possible Denial of Service attack. More info on the vulnerability is available on Technet. I know some people have had an issue applying this patch and with any luck this post can save you a bit of time.
The Problem
When the patch is pushed through something like SCCM, automatic updates, or WSUS, the it seems to have trouble installing on Lync Server 2010 Edge Servers.
Symptoms
External users were able to connect just fine, but when trying to send an IM to a user, it was unable to connect to the server. Lync Server 2010 Director servers logged an event 14502 as pictured below, indicating they were unable to communicate with the Edge Servers.
Event ID 14502 on Lync Server 2010 Director Servers after installing KB2982385
The Event Logs in the Edge Servers log errors KB2982385 failed to install, and that Lync services failed to start.
Solution
To correct this, all you have to do is to manually install KB2982385, either from Windows Update or manually download it here from Microsoft Download Center.