diff --git a/docs/commandref.html b/docs/commandref.html
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--- a/docs/commandref.html
+++ b/docs/commandref.html
@@ -1381,21 +1381,25 @@ A line ending with \ will be concatenated with the next one, so long lines
     <br>
     Defines a calendar device.<br><br>
 
-    A calendar device periodically gathers calendar events from the source calendar at the given URL. The URL must
-    start with <code>http://</code>, not <code>https://</code>, and the file at the given URL
+    A calendar device periodically gathers calendar events from the source calendar at the given URL. The file at the given URL
     must be in ICal format.<br><br>
 
-    Note for users of Google Calendar: You can literally use the private ICal URL from your Google Calendar with the
-    <code>https://</code> replaced by <code>http://</code>. Google App accounts do not work since requests to the URL
-    get redirected first and the fhem mechanism for retrieving data via http cannot handle this. Neither can it
-    handle HTTPS.<br><br>
+    If the URL
+    starts with <code>https://</code>, the perl module IO::Socket::SSL must be installed
+    (use <code>cpan -i IO::Socket::SSL</code>).<br><br>
+
+    Note for users of Google Calendar: You can literally use the private ICal URL from your Google Calendar.
+    Google App accounts do not work since requests to the URL
+    get redirected first and the fhem mechanism for retrieving data via http/https cannot handle this. If your Google Calendar
+    URL starts with <code>https://</code> and the perl module IO::Socket::SSL is not installed on your system, you can
+    replace it by <code>http://</code>.<br><br>
 
     The optional parameter <code>interval</code> is the time between subsequent updates
     in seconds. It defaults to 3600 (1 hour).<br><br>
 
     Examples:
     <pre>
-      define MyCalendar Calendar ical url http://www.google.com/calendar/ical/john.doe%40example.com/private-foo4711/basic.ics
+      define MyCalendar Calendar ical url https://www.google.com/calendar/ical/john.doe%40example.com/private-foo4711/basic.ics
       define YourCalendar Calendar ical url http://www.google.com/calendar/ical/jane.doe%40example.com/private-bar0815/basic.ics 86400
       </pre>
   </ul>