I had the same problem recently, but the cause was actually quite different. In my case it was a proxy issue - I had somehow managed to create some environment variables on my user profile for http_proxy and https_proxy, which Jabber used when trying to connect to the CUCM server. The proxy then disallowed this traffic and Jabber showed the "Cannot communicate with the server" message.
The csf-unified.log showed the following lines:
2015-09-18 12:21:30,510 INFO [0x00001384] [ls\src\http\BasicHttpClientImpl.cpp(410)] [csf.httpclient] [http::executeImpl] - *-----* HTTP response from: https://<my_cucm_machine>:8443/cucm-uds/version  -> 0. 2015-09-18 12:21:30,510 ERROR [0x00001384] [ls\src\http\BasicHttpClientImpl.cpp(414)] [csf.httpclient] [http::executeImpl] - There was an issue performing the call to curl_easy_perform: UNKNOWN_ERROR
I had curl installed on my (Windows) machine, so I was able to reproduce the issue by opening a bash shell and running
curl -v https://<CUCM machine>:8443/cucm-uds/version
This showed the connect() attempt to the proxy server and returned an response of 'Received HTTP code 403 from proxy after CONNECT'.
You can check for this by looking in the HKEY_CURRENT_USER/Environment registry key, or by right-clicking My Computer->Properties, then Advanced system settings->Advanced tab->Environment Variables. (You can also run set from a command prompt, but that won't show whether it's user or system level.)
Deleting the http_proxy and https_proxy entries fixed up the issue for me.
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