The CalDAV sync between Team Calendar for Confluence Data Center and Third-Party calendar is not possible when Allow basic authentication on API calls in Confluence is disabled.

This because the Team Calendar app still rely on using Basic Authentication (username and password) for authentication as mentioned in Creating an allowlist when basic authentication is disabled:

Some Atlassian products rely on using username and password for authentication in API calls. Here's a list of URLs we recommend that you add to your allowlist to ensure smooth functioning of some Atlassian products:

Confluence

• To use Team Calendars:
  • /plugins/servlet/team-calendars/caldav/*
  • /rest/calendar-services/*
• To use WebDAV:
  • /plugins/servlet/confluence/default
  • /plugins/servlet/confluence/default/*
• To use RSS feeds:
  • /createrssfeed.action
  • /spaces/createrssfeed.action
  • /feeds/*

To enhance the security, it is best to add support for Personal Access Token (PAT) when using CalDAV to synchronise the Team Calendar for Confluence Data Center with Third-Party Calendar.