Roundcubeplus CalDav: no valid Ical Data found

I would like implement a calendar on my webpage. We also moved away from Google and co. That’ s why we use the calendar which comes with RoundcubePlus. It’ s working with our iPhones but when we fill in all the required data in the settings of the plugin, we get the following content (copied out of the cron.log):

Log created on 20:04:20 08:07:17
CalendarURL:https://caldav.example.com/
using temp file: /var/www/tmp/result.txt for Server Response^M
<?xml version=”1.0″?>^M
<d:multistatus xmlns:d=”DAV:” xmlns:s=”https://sabredav.org/ns&#8221; xmlns:cal=”urn:ietf:params:xml:ns:caldav” xmlns:cs=”https://calendarserver.org/ns/”/&gt;^M
Lines processed: 2^M
WARNING: no valid Ical Data found in Server Response !^M

Any help would be greatly appreciated!