From Zarafa wiki
When having problems with Z-Push you most likely want to check for errors. This can be done by enabling debugging from within Z-Push.
- The first thing you will need to do is to create a file called debug.txt inside the Z-Push document root.
- Make sure this file is writeable by your web server.
- Optional: If you also want to see the XML data inside the debug file, you will need to set the option WBXML_DEBUG to true inside the file wbxml.php.
When you are done debugging it is advised to revert back the changes as the debug file can grow in size really fast.
This applies to Z-Push versions 1.5.x