Nimbostratus
Nimbostratus@sipple31 Did you ever get this resolved? Having the same issue.
Actually my boss and I sat in a conference call for two hours last night with Microsoft and F5 engineers. They ran through our environment and all looked OK. Long story shortened... if you put your reverse proxy virtual server type to "Performance (layer 4)" and everything works, you are running into the same issue as us.
Basically it seems that SOME (notice I didn't say all) parts of Lync are unhappy with how the F5 is chaining or terminating the certs. We even tried putting a cert from our corporate CA on the outside edge of the F5, trusting it, and still wouldn't work. Definitely something F5 needs to work out.
They opened up an engineering ticket for us last night - I can let you know what happens.