It should be a bug for z-push version 22.214.171.124 currently present on WebTop5.
Try this workaround and let me know if I can solve it.
Unpack it in an empty folder.
In your z-push, find the "include" folder: rename iCalendar.php in iCalendar.php.old.
This is precisely the file that generates the error in the log that you indicated, carried by awl in z-push, without having carried all the other php employees.
Then copy from the awl package, the contents of "inc" (there are various php: iCalendar.php, XMLElement.php ...) inside z-push's "include".