I am using DAVx5 to sync CalDav Tasks to my Android phone and have been getting an error back from my Kolab installation that was pointing to a DateTimeZone problem. Error report in /var/log/iRony/errors.log is:

{main} in /usr/share/roundcubemail/plugins/libkolab/lib/kolab_storage_cache.php on line 1079 (REPORT /iRony/calendars/<<my account>>/9d34182a-c431-4876-9e60-7bcde927d1bf/)
[20-May-2021 15:50:21,419258 +0100]: PHP Error: DateTimeZone::__construct(): Unknown or bad timezone () (error 500)

Changing line 1079 of kolab_storage_cache.php from:

$object[$prop] = new DateTime($object[$prop]['dt'], new DateTimeZone($object[$prop]['tz']));

to:

$object[$prop] = new DateTime($object[$prop]['dt'], new DateTimeZone('Europe/London'));

provide a (temporary) fix for me but I believe should be fixed properly.