Launch Pidgin, add an account and set it up to connect to your BOSH endpoint. In Advanced tab, specify your BOSH URL there. Use the URL that will be used by Roundcube to connect. In most cases this URL will have the same hostname as your Roundcube installation.
If that does not work, you need to work on that first. See log files of your XMPP server and your proxying web server. Consult the available documentation and support channels of your XMPP server and/or web server.
Launch Chrome, hit F12 (developer tools), go to Network tab, select XHR (on the bottom). BOSH connections are ones that have path like 'http-bind' (or whatever your BOSH endpoint is).
Clicking on it will reveal detailed information on the right pane. What interesting to us are the request (Headers tab, section 'Request Payload') and the response (the Response tab). Make sure you provide us with these information on bug reports.