When accessing the preview of a calendar %baseurl%/calendar/previewcalendar.action?subCalendarId=%calendarid% something that wants to be a feature tour shoes up but Somethings seem to be missing. AFAICS this behavior is new since from 5.3.10 to 5.3.11

Steps to replicate:

1. Install confluence 5.8.18
2. Install Team calendar 5.2.18
3. Create a Calendar by subscribing to any calendar (exp: https://calendar.google.com/calendar/ical/en.danish%23holiday%40group.v.calendar.google.com/public/basic.ics)
4. Leave the related space empty
5. Click on the calendar, embed/share you will noticed the url as below: http://localhost:5818/calendar/previewcalendar.action?subCalendarId=acb71ec9-2e1d-4e80-a106-6304e46d5e66 which working fine if you opening it at a new tab
   
6. Upgrade to latest Team Calendar
7. Click on the embed url again, and open in a new tab:
8. Noticed that the preview broken and load forever
   

Workaround

1. Edit the calendar
2. Fill in the related Space
3. Check on the embed again, noticed the URL has been changed as below, which is working fine on a new tab:
   http://localhost:5818/display/WAYN/calendar/acb71ec9-2e1d-4e80-a106-6304e46d5e66?calendarName=sub