I think the first thing you should do is contact your host. Something definitely changed with the PHP configuration. The original issue was that enabling zlib compression in the control panel led to a blank page.
Now, the reason your shoutbox isn't working seems related. The XML that is used is being invalidated because there is a blank line at the beginning. If you haven't changed any of the actual templates recently I'd definitely contact them, as things like this don't just start happening unless something was altered.
