Solution for shared hosting users
add this to plugins\libcalendaring\libvcalendar.php Dont forget to replace xxxxx with yours paths or correct to yours. If having problem uncomment echo and check for path errors.
spl_autoload_register(function ($class) {
if (strpos($class, 'Sabre\\')===0) {
$classs = str_replace('\\', '/', $class);
$classs = str_replace('Sabre/VObject/', 'vobject/lib/', $classs);
$classs = str_replace('Sabre/Xml/', 'xml/lib/', $classs);
$file = '/home/xxxxx/domains/xxxxx/public_html/plugins/libcalendaring/vendor/sabre/'.$classs.'.php';
include $file;
//echo $file;
}
});