Get the following error when following a calendar preview link (<server>/calendar/previewcalendar.action?subCalendarId=<guid>) and not being authenticated:

java.lang.NullPointerException
at com.atlassian.confluence.extra.calendar3.calendarstore.AbstractCalendarDataStore.hasViewEventPrivilege(AbstractCalendarDataStore.java:396)

Should lead to the login screen (leading to the cal preview afterwards) as page links with permission restrictions do.