Unable to accept shared calendar invitations from Exchange On-Premises to Exchange Online in a hybrid environment

Stadelmann, Enzo 0 Reputation points
2026-09-24T08:12:36.2433333+00:00

Hello,

We are experiencing an issue with calendar sharing in our Exchange hybrid environment (Exchange On-Premises and Exchange Online).

Problem description

An Exchange On-Premises user creates a secondary calendar and shares it with colleagues whose mailboxes are hosted in Exchange Online.

The recipients receive the sharing invitation, but when they click the "Accept" button in Outlook Classic, the following error appears:

"There was a problem connecting you to the shared calendar. Please ask the person who invited you to send another sharing invitation."

The issue affects multiple recipients, including myself, and is not limited to a single Outlook profile or workstation.

Environment

  • Calendar owner: Exchange On-Premises (UserMailbox).
  • Recipients: Exchange Online (UserMailbox).
  • Outlook version tested: Microsoft Outlook for Microsoft 365 MSO, Version 2608 (Build 16.0.20326.20072), 64-bit.
  • Exchange hybrid environment with Organization Relationships enabled.

Troubleshooting already performed

  1. The calendar owner created a new secondary calendar and sent new sharing invitations. The same error occurred.
  2. Multiple Exchange Online users, including myself, tested the invitation and encountered the same error.
  3. We verified the calendar folder permissions using Exchange PowerShell. The recipients have the appropriate permissions (Editor) on the shared calendar.
  4. The reverse sharing scenario works: an Exchange Online user can create a secondary calendar, share it with the On-Premises user, and the latter can access it.
  5. We tested the invitation in Outlook Web, but the "Accept" button is not displayed.
  6. We verified the Exchange Online Organization Relationships. They are enabled, with FreeBusyAccessEnabled set to True and FreeBusyAccessLevel set to LimitedDetails.

The issue therefore appears to be specific to sharing calendars from Exchange On-Premises to Exchange Online.Hello,

We are experiencing an issue with calendar sharing in our Exchange hybrid environment (Exchange On-Premises and Exchange Online).

Problem description

An Exchange On-Premises user creates a secondary calendar and shares it with colleagues whose mailboxes are hosted in Exchange Online.

The recipients receive the sharing invitation, but when they click the "Accept" button in Outlook Classic, the following error appears:

"There was a problem connecting you to the shared calendar. Please ask the person who invited you to send another sharing invitation."

The issue affects multiple recipients, including myself, and is not limited to a single Outlook profile or workstation.

Environment

  • Calendar owner: Exchange On-Premises (UserMailbox).
  • Recipients: Exchange Online (UserMailbox).
  • Outlook version tested: Microsoft Outlook for Microsoft 365 MSO, Version 2608 (Build 16.0.20326.20072), 64-bit.
  • Exchange hybrid environment with Organization Relationships enabled.

Troubleshooting already performed

  1. The calendar owner created a new secondary calendar and sent new sharing invitations. The same error occurred.
  2. Multiple Exchange Online users, including myself, tested the invitation and encountered the same error.
  3. We verified the calendar folder permissions using Exchange PowerShell. The recipients have the appropriate permissions (Editor) on the shared calendar.
  4. The reverse sharing scenario works: an Exchange Online user can create a secondary calendar, share it with the On-Premises user, and the latter can access it.
  5. We tested the invitation in Outlook Web, but the "Accept" button is not displayed.
  6. We verified the Exchange Online Organization Relationships. They are enabled, with FreeBusyAccessEnabled set to True and FreeBusyAccessLevel set to LimitedDetails.

The issue therefore appears to be specific to sharing calendars from Exchange On-Premises to Exchange Online.
Thank you in advance for your assistance.

Exchange | Hybrid management
Exchange | Hybrid management

The administration of a hybrid deployment that connects on-premises Exchange Server with Exchange Online, enabling seamless integration and centralized control.

1 answer

Sort by: Most helpful
Most helpful Newest Oldest
  1. 30432239 0 Reputation points
    2026-09-24T08:21:05.79+00:00

    Hi Enzo,

    Based on the symptoms you described, this looks more like a hybrid calendar-sharing/configuration issue than an Outlook client issue.

    Since the reverse scenario (Exchange Online → Exchange On-Premises) works, and multiple Exchange Online users are getting the same error when accepting a calendar shared from On-Premises, I would focus on the hybrid configuration and the way the sharing invitation is being generated.

    A few things I would check:

    Verify the Organization Relationship in both directions. Free/busy working correctly does not necessarily mean that calendar sharing is fully supported. The Organization Relationship settings mainly control availability information, while calendar folder sharing involves additional Exchange functionality.

    Check the sharing policy on the On-Premises Exchange server. Run:

    Get-SharingPolicy | fl Name,Enabled,Domains

    Also check which sharing policy is assigned to the affected mailbox:

    Get-Mailbox ******@domain.com | fl SharingPolicy

    Check the actual calendar folder permissions from the On-Premises side.

    Get-MailboxFolderPermission ******@domain.com:\Calendar

    Since this is a secondary calendar, make sure you are checking the exact folder path/name rather than only the default Calendar.

    Check whether the invitation contains the correct Exchange sharing information. I would also inspect the original sharing invitation/message headers. In a hybrid environment, an incorrect or missing Exchange sharing endpoint/URL can result in Outlook showing the exact error you are seeing when clicking Accept.

    Verify the hybrid configuration. If the environment was configured or upgraded some time ago, I would run the latest Hybrid Configuration Wizard again and verify the OAuth/hybrid configuration. In particular, check that the OAuth relationship and Exchange servers participating in hybrid are healthy.

    You can also test whether the problem is specifically related to the sharing invitation by manually adding the shared calendar in Outlook using Add Calendar → From Address Book after the permissions have been assigned. If that works while accepting the invitation fails, it would strongly point toward an issue with the sharing invitation rather than the calendar permissions themselves.

    One other important point: FreeBusyAccessEnabled = True only confirms that cross-premises availability is configured. It does not by itself confirm that cross-premises folder sharing/acceptance of sharing invitations is working correctly.

    Given that the issue is reproducible for multiple Exchange Online users and only occurs in the On-Premises → Exchange Online direction, I would concentrate on the hybrid OAuth/sharing configuration and the sharing invitation generated by the On-Premises Exchange server rather than troubleshooting individual Outlook profiles.

    If possible, I would also run Test-OAuthConnectivity from both sides and check the Exchange event logs/message tracking around the time the invitation is accepted. That should help identify whether the request is failing during authentication, autodiscover, or access to the shared folder.

    Was this answer helpful?

    0 comments

Your answer

Answers can be marked as 'Accepted' by the question author and 'Recommended' by moderators, which helps users know the answer solved the author's problem.