Hi Mike,
Check the users' application logs: when communicator fails to log in, it usually drops an error message in the application log that gives more details as to why it wasn't able to connect.
Two of the most common causes of failing to log in are problems with DNS and Certificates.
My guess is that it is DNS: if you have LCS up and running right now, you probably have 1 SRV record pointing to the LCS pool. When the new OCS client comes up, it looks for the SRV record in DNS and sees the LCS pool - it tries to log in to LCS, but it can't because the new OC clients aren't supported with LCS.
But check the error logs & post what you find - that will help narrow down the cause.
Regards,
Matt