There is a bug (or, rather, an enhancement needed) in CalDAVClient.php
When connecting to EGroupWare, you get the error:
Exception: Could'n find a CalDAV-collection under the url
It is an easy fix.
The problem is that EGW sends header of "Dav:" not "DAV:" as in this code.
Change around line 168 from
$dav_header = preg_grep('/^DAV:/', $headers);
to
$dav_header = preg_grep('/^DAV:/i', $headers);
and around line 171 from
$dav_header = preg_replace('/^DAV: /', '', $dav_header);
to
$dav_header = preg_replace('/^DAV: /i', '', $dav_header);
(allow upper/lower case on the preg_ functions)
After that, you get data from EGW!