Multitenant Org Web Outlook Calendar Sharing Issue

Mark Bewley 71 Reputation points
2025-08-06T18:30:09.1533333+00:00

We have two tenants in a multitenant org. Viewing user's calendars is working in one direction but not the other.

In the tenant having the issue, most users prefer to use the web version of Outlook. Using the DevTools these messages are logged:

service.vnc?action=GetCalendarFolders&app=Calendar&n=42

200: Success

service.vnc?action=GetCalendarFolderConfiguration&app=Calendar&n=12

service.vnc?action=GetCalendarFolderConfiguration&app=Calendar&n=13

Error 500: Internal Server Error

Response Text: "You don't have permission to open this mailbox".

Unfortunately I am not an admin in the tenant they are trying to access the calendar in. As everything looks to be correctly configured our IT Support is not that interested in investigating the problem.

We previously had a trust between the two tenants and used the mail contacts method to allow calendars to be shared between the two tenants - this worked well. But this was stopped so we could migrate over to MTO (to get the benefit of syncing, as manually managing the contacts was tiresome).

Outlook | Web | Outlook on the web for business | Calendar
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  1. Chris Duong 1,480 Reputation points Microsoft External Staff Moderator
    2025-08-06T19:15:16.9766667+00:00

    Hi @Mark Bewley,  

    Good day to you! Welcome to Microsoft Q&A forum. 

    To understand the situation and be able to offer you relevant suggestions, we would need a little more information from you. Kindly help us in providing the following information.  

    May I double confirm that you checked with Admin if your account still has License? To check this kindly sing into https://admin.microsoft.com/ with Admin credentials--> choose Active User under Users-->click on your username--> choose Licenses and apps-->Expand Apps--> Make sure that Exchange Online(Plan1 or 2) is selected. If your account has License kindly uncheck it and reapply it then wait the sync period to see if it will make any difference  User's image

    User's image If you don't have Admin privilege, kindly contact your organization Admin to check above for you.  

    After performing license checks or role adjustments, it’s also important to clear your browser cache, as outdated session data may interfere with updates. 
    You may also refer to Getting a mailbox not found error in Outlook on the web - Microsoft 365 admin | Microsoft Learn.  

    If your user account is assigned too many admin roles, it may cause unexpected behavior. To check this: 

    1. In the Admin Center, go to Users > Active users
    2. Click your username > scroll to Roles > click Manage roles
    3. Remove all roles except Global Administrator, if applicable. 
    4. Click Save changes and try accessing again after clearing your browser cache

    If the issue still present, you may also (use admin account to) go to Office 365 Admin Center-> Support-> New Service Request. The support engineers there have the correct escalation channel, and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket: Ways to contact support for business products - Admin help. The engineers in related team have higher permission and resources than us to help check the root cause from background directly.  

    We look forward to your response. Kindly note that the above requested information is required for further analysis and troubleshooting of your situation. Your patience and cooperation are much appreciated. Have a nice day!  


    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.  

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  2. Chris Duong 1,480 Reputation points Microsoft External Staff Moderator
    2025-08-08T19:53:47.6733333+00:00

    Hi @Mark Bewley,   

    Hope things are running smoothly on your end. 

    Following up on the support thread we've been working on. I hope the information I shared in my previous answer resolve the issue you were facing. If you are still facing the same issue or need assistance with anything else, kindly respond to this email, and I will be happy to help. 

    We want to make sure everything is working as expected and that your experience remains uninterrupted. 

    Thank you for your patience and understanding throughout the troubleshooting process.  

    I look forward to hearing from you soon!


    If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".   

    Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.  

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  3. Chris Duong 1,480 Reputation points Microsoft External Staff Moderator
    2025-08-12T18:54:37.7+00:00

    Hi @Mark Bewley,    

    I hope you're having a good day so far. 

    I'm reaching out again to follow up on your issue and check if everything has been resolved. My goal is to ensure your experience remains smooth and hassle-free. If you're still encountering any problems or have run into new challenges, please let me know which steps you're currently stuck on and happy to provide further help whenever you need it.  

    If you have any further questions, feel free to tag me in your reply so I can assist you directly. 

    Looking forward to your update! 


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