This comes second hand from a work college but I figured I should post it since there is nothing on the web about this issue.
Basically as it is described to me, Lync client for Mac OSX (10.6 and 10.7) will connect, show presence, but within one minute it blanks out and disconnects with an error.
Here is a sample error log from a TechNet Forums post where I got the solution from:
2011/12/22 10:46:04.328 [o365] process server certificate from input token failed
So there isn’t a fix just yet however there is a work around. This seemed to resolve the issue, thanks to "Jonathanpisarczyk" on the forums:
Open Lync Control Panel -> security
in the right pane we can find the authentication methods
Double click on the global setting so that we can edit that.
Uncheck the Enable Certificate Authentication.
Commit the changes.
User will need to signoff and back on.
If you have any other details on resolutions please leave them below in the comments.
UPDATE: Ok here some new info. This fixed us only for a few days then we had the problem again. We then fixed it by re-enabling the certificate based authentication. Some posts claim post CU4 you should keep it enabled. Perhaps turning it off and back on fixes it? My college likes to say "jiggle the toilet handle"