What to do if you receive this error:
Could not connect: <stream:error xmlns:stream="http://etherx.jabber.org/streams"><service-unavailable xmlns="urn:ietf:params:xml:ns:xmpp-streams"/></stream:error>
Check if you have secure set/unset on this page (System->Security->Settings).
Check what you have set for your httpbinding path. (System->Service Parameters -- your server, Cisco XCP Web Connection Manager).
Finally, check your http binding URL (this example is for a secure connection):
If you have Enable Web Client to IM/P Service Secure Mode checked, and the config above is properly formed, then you're probably running into a security certificate problem. You can solve it (temporarily) by using Firefox to go to https://<your server>:7335/httpbinding, and then do the whole Firefox certificate exception thing. Then reload the basic chat demo in Firefox and try again. You should be able to log in now.
If you are using Chrome, and have not added a permanent exception, then simply go to the BOSH URL with Chrome and see what happens. If you see a security message in Chrome when you go to a URL like this:
...then that tells you Chrome doesn't trust the certificate.