You can't add an Internet calendar in Outlook 2019, Outlook 2016, Outlook for Microsoft 365, and new Outlook

Original KB number:   4025591

Symptoms

You try to add an Internet calendar in Microsoft Outlook 2016, Outlook 2019, or Outlook for Microsoft 365 by following these steps:

1. Switch to the Calendar module.
2. In the Manage Calendars group on the toolbar, select Open Calendar.
3. Select the From Internet option.
4. Add the path to your Internet calendar file.
5. Select OK.

or

you try to add an Internet calendar in new Outlook for Windows by following these steps:

1. Switch to the Calendar module.
2. Select Add Calendar from the navigation menu.
3. In the Add Calendar window, select Subscribe from web in the navigation pane.
4. Add the path to your Internet calendar file.
5. Select Import.

When you use either of these methods for some Internet servers, the calendar isn't added successfully. After a progress window opens briefly, you're returned to the calendar module without a new calendar added. Additionally, you don't receive any error message.

Cause

By default, Outlook 2016, Outlook 2019, Outlook for Microsoft 365, and new Outlook use a new modern authentication stack. Even if the Internet calendar file that you try to open doesn't require authentication, Outlook broadcasts its ability to do modern authentication on the server that contains the calendar. Most server software responds to this broadcast by supporting modern authentication. However, some servers consider the broadcast to be an invalid client request. Therefore, they return a response that causes Outlook to cancel the attempt to add the calendar.

For example, server software that's running on Amazon Web Services (AWS) mighty consider the authentication broadcast to be invalid and refuse the connection.

Workaround

To work around this issue, use Outlook on the web. This app doesn't have the authentication header behavior described in the Cause section. Therefore, it opens the Internet calendar that you want to add.

To use Outlook on the web to add an Internet calendar, follow these steps:

1. Sign in to Outlook on the web.
2. Switch to the Calendar module.
3. Select the Add calendar menu.
4. Select the From internet option.
5. Add the path to your Internet calendar file and an optional calendar name.
6. Select Save.

More information

When you add an Internet calendar by using Outlook on the web, the server that's running Exchange Server manages the calendar subscription and updates. Calendar updates sync approximately every four hours.

The calendar that's added from Outlook on the web doesn't appear in the Outlook desktop application as an Internet calendar. Instead, it's displayed as a calendar entry in the Other Calendars group. Because no subscription information is handled or controlled by the Outlook desktop application, you don't have to configure any client-side Internet calendar options for the calendar that's added through Outlook on the web.

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