Problem

When subscribing to Google Calendars (instructions: Subscribe to Google Calendar from Team Calendars), the error 'Specified Calendar does not seem like iCalendar' is shown in the dialog box.

Diagnosis

Diagnostic Steps

1. Enable Team Calendars for Confluence Debugging by adding com.atlassian.confluence.extra.calendar3 class with DEBUG level (instructions)
2. Download the .ics file by navigating to the URL address generated in Google Calendar Private Address ICAL
3. Add new Team Calendars for Confluence (see Create, Add, and Edit Calendars), choose Import Calendar to choose the .ics file generated in Step 2

4. In atlassian-confluence.log, you'll see this error:

   2015-03-17 22:06:32,687 DEBUG [http-bio-10564-exec-10] [extra.calendar3.calendarstore.SubscriptionCalendarDataStore] validate Unable to interpret response from https://www.google.com/calendar/ical/local.tester%40localhost.com/private-6afddd89d3427dbc0036ded2e229f906/basic.ics as iCalendar
    -- referer: http://localhost:10564/confluence/calendar/mycalendar.action | url: /confluence/rest/calendar-services/1.0/calendar/subcalendars.json | userName: admin
   net.fortuna.ical4j.data.ParserException: Error at line 8,790:null

5. Open the .ics file in Text Editor (e.g: Notepad++) to find the line that's throwing the error (see the error in Step 4)

6. The sample of the offending entry in .ics file:

   Sample Content

   URL:http://maps.google.de/maps?q=CinemaxX+Essen&hl=de&ie=UTF8&ll=51.4595868
    12593884%5C\,7.0040953159332275

   Sample Content

   URL:http://HOST:8090/download/attachments/4063626/Janu
    ary%2014\,%202015.xlsx?api=v2

7. Find the event name that's located few lines below the offending entry (it's the value of parameter called "SUMMARY:" that you find)
8. Go to the Google Calendar and find the event in Step 7 to fix the URL

Cause

The old Google Maps URL in the Calendar Event is causing NullPointerException when imported to Team Calendars for Confluence. This error can also be seen when there is a reference to a Team Calendar event within a Google Calendar event.

Resolution

1) Modify the Google Maps URL in the Google Calendar Event. (Try to put the old URL in the browser to get the new URL syntax)

2) Go to the Google Calendar event and remove the reference to the Confluence Team Calendar event (in this case an XLSX reference)

Alternative Cause

An error like the following could produce from the debug logging:

2022-09-28 13:58:45,374 DEBUG [http-nio2-8091-exec-84 url: /rest/calendar-services/1.0/calendar/subcalendars.json; user: confluenceuser] [extra.calendar3.calendarstore.SubscriptionCalendarDataStore] validate Unable to interpret response from webcal://confluence.pagerduty.com/private/123456789/feed/0NDPPAD as iCalendar
 -- referer: https://confluence.com/pages/editpage.action?pageId=966614588 | url: /rest/calendar-services/1.0/calendar/subcalendars.json | traceId: 4e83f397bc4840c8 | userName: confluenceuser
net.fortuna.ical4j.data.ParserException: Error at line 857:Components nested too deep
	at net.fortuna.ical4j.data.CalendarParserImpl.parse(CalendarParserImpl.java:162)
	at net.fortuna.ical4j.data.CalendarBuilder.build(CalendarBuilder.java:183)
	at net.fortuna.ical4j.data.CalendarBuilder.build(CalendarBuilder.java:171)
	at com.atlassian.confluence.extra.calendar3.calendarstore.SubscriptionCalendarDataStore.validate(SubscriptionCalendarDataStore.java:212)
	at com.atlassian.confluence.extra.calendar3.calendarstore.CalendarDataStoreCachingDecorator.validate(CalendarDataStoreCachingDecorator.java:115)
	at com.atlassian.confluence.extra.calendar3.calendarstore.DelegatingCalendarDataStore.validate(DelegatingCalendarDataStore.java:72)
	at com.atlassian.confluence.extra.calendar3.DefaultCalendarManager.validateSubCalendar(DefaultCalendarManager.java:328)
	at com.atlassian.confluence.extra.calendar3.rest.resources.CalendarResource.updateSubCalendarInternal(CalendarResource.java:445)

This error indicates Team Calendars was unable to read the calendar completely (possibly terminating mid ICS download). PagerDuty mentions this in their documentation as being related to the length of usernames or schedule being longer than 28 characters:

Atlassian expects the name of your schedule in PagerDuty to be 28 characters or fewer. If the name of your PagerDuty schedule has 29 or more characters, then you will see the following error in Confluence: Specified calendar does not seem like iCalendar. Note that this refers to the schedule's name and not the iCal URL.

The same error message will appear if one or more of your users in PagerDuty has a name that is 29 or more characters long.

Source: Schedules in Third-Party Apps - PagerDuty