Microsoft's patch released today, KB2862772 (MS13-059), is apparently intended to address this issue. The problem itself was a result of the July patch changing the way IE handles JavaScript and Session Handles.
Really, the issue was Microsoft's to fix, not Symantec's, as it affected more than just Web Messenger, but there have also been confirmed cases of other JS apps being affected as well.
I'm working with some of my customers to confirm if this fixes the issue or not.