I came across this funny issue when my client is trying to download/synchronize SharePoint calendar events to his MS Outlook.
The calendar instance is added successfully into his Outlook. However, all the events are not populated and the following message shown in the outlook status bar.
Execute the following script via SharePoint Powershell. Kindly change the calendarName to your calendar name.
#Here we go.
$web = Get-SPWeb https://yoursite
$calendarName = "My Calendar"
$calendar = $web.Lists[$calendarName];
$calendar.ExcludeFromOfflineClient = 0;
This script simply enable the calendar to be available for offline client download.
Optionally, you can go to your Calendar’s List Setting > Advanced Setting and turn “Offline Client Availability ” on.
Go back to your Outlook, Hit “Shift” + F9 to refresh the calendar.