An issue came up lately where a clients site collection was moved from one server to another. The client had set up some SharePoint to Outlook connections to some calendar lists on their original server. These connections no longer functioned in Outlook after this move. Read more on this functionality here
Here is the steps to resolve the calendar issue.
Errors seen in the Outlook when attempt to sync
“Task ‘SharePoint’ reported error”
“Outlook cannot connect to the SharePoint List”
“The server may not be reachable from your location.”
Notes of Analyse
• A calendar connection is established from the SharePoint side and not from Outlook.
• The server location has changed because of the forced move to Project Hosts.
• I investigated the possibility of changing the connection detail within Outlook. It appears not to be possible.
• Therefore to resolve this we must remove the calendar in Outlook and then re-establish the connection.
• *Any Items which had been added to the calendar on the Outlook side since it stopped synchronising, are in danger of being lost unless these appointments added to the SharePoint calendar before we re-establish the connection.
1. Complete the * task above first.
2. In Outlook choose File then choose click Account Settings
3. Choose the SharePoint Lists tab and then choose the list name like below (Calendars are classified as lists in SharePoint).
4. Finally Choose Remove
5. Reference your calendars in Outlook and note that the calendar has been removed.
6. Now simply return to the calendar list in SharePoint and choose Connect to Outlook
7. You may receive a similar message as below.
That should now resolve the issue.
Ref : Bright Work Support