You try to add an Internet calendar in Microsoft Outlook 2016, Outlook 2019 or Outlook for Office 365 by following these steps:
- Switch to the calendar module.
- In the Manage Calendars group on the toolbar, click Open Calendar.
- Select the From Internet option.
- Provide the path of your Internet calendar file.
- Click OK.
When you use this method for some Internet servers, the calendar is not added successfully. After a progress window opens briefly, you are returned to the calendar module without a new calendar added. Additionally, you don't receive any error message.
By default, Outlook 2016, Outlook 2019 and Outlook for Office 365 use a new modern authentication stack. Even if the Internet calendar file that you are trying to open does not 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) may consider the authentication broadcast to be invalid and refuse the connection.
To work around this issue, use Outlook Web App (OWA). OWA doesn't have this authentication header behavior. Therefore, it will successfully open the Internet calendar that you want to add.
To use OWA to add the calendar, follow these steps:
- Sign in to OWA.
- Switch to the Calendar module.
- Click the Add calendar menu.
- Select the From internet option.
- Provide the URL and an optional calendar name.
- Click Save.
When you add the Internet calendar by using OWA, 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 OWA does not appear in the Outlook desktop application as an Internet calendar. Instead, it is 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 OWA.