Hi Manolo,
Unfortunately Apple closes the connection for a variety of reasons if something's not set up correctly and doesn't provide a good reason. An invalid certificate or, say, a sandbox certificate used on production (or vice versa) could cause this, as well as other things. Check that your certificates are correct and that your push service provider is set up correctly.
In order to help you better, it would help to have (1) the commands you used to set up the push service provider and try to send the push, and (2) any additional logs from uniqush-push