Setup Failure Codes/PHP XML support
The page identifies a critical configuration problem. DAViCal will not work at all until this is resolved.
The caldav protocol is XML, even though the actual calendar events are not. DAViCal needs the PHP XML extension to parse what a client wants it to do.
The XML extension was part of the core PHP package in PHP 5, but has been split out for PHP 7, so e.g. on Debian systems you'll have to install the php-xml package now.