ICS/HTML published calendar returns 500 error

Question

ICS/HTML published calendar returns 500 error

YMO 15

We are facing an issue with ICS calendar publishing in Exchange Online.

The problem seems to have been started after we changed the users’ login domain (and Default domain for the tenant) from @ABC to @ABD and assigned EMS A3 Licenses for onboarding PC in Intune and Hello Enterprise.

For affected users, the published calendar .ics link returns a 500 Internal Server Error when it's used trough Google Agenda, Teamup, or linking the calendar in OWA... while if we use a browser we can download the ics file or .html link works fine.

This issue only happens with existing users migrated to the new domain.

For newly created users, ICS publishing works correctly (even after changing their UPN to the new domain).

We have already checked mailbox folder permissions, sharing policies, re-enabled OWA, and unassigned EMS A3 licenses. Nothing solved the issue.

Has anyone experienced this before or knows how to force a repair/reset of the ICS publishing feature for existing mailboxes?

Troubleshooting already performed:

Verified and recreated PublishedCalendarUrl and ICS links via Pwershell and OWA
Checked sharing policy → Anonymous:CalendarSharingFreeBusyReviewer.
Reset folder permissions on calendar folder
Re-enabled OWA
Tested with and without custom OWA Mailbox Policy.
Created new calendar folder and attempted publishing → same issue for not working accounts.
Verified EmailAddresses still include both new and legacy SMTP aliases and the primary is correct.

Compared working ICS or HTML requests using Invoke-WebRequest from new users (returns valid VCALENDAR 200 OK) vs. old users (500 error). Ex :

Outlook_32x

500

Something went wrong.

Outlook is not supported on this browser. Not all web browsers support Outlook on the web. Find a supported browser or logout here. Click here to sign out.

More details clientIPAddress XXXXXXXXXXXXXXX X-ClientId: 2B9EA3428E684D5CA0A5FF1450991CA9

X-OWA-Error Microsoft.Exchange.Clients.Owa2.Server.Core.OwaBasicUnsupportedException

X-OWA-Version 15.20.9052.14

X-FEServer AM0PR02CA0156

X-BEServer AM9PR04MB7636

Date:8/21/2025 1:29:15 PM

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Answer 1

Kudos-Ng 15,050 Microsoft External Staff Moderator

Hi YMO,

Thank you for posting your question in the Microsoft Q&A forum.

From what you've described, it seems you've thoroughly investigated the issue, yet the problem persists specifically for existing users who were migrated to the new domain. This behavior is quite unusual and suggests that something may have gone wrong during the domain change process for those users.

I did some additional research and came across a recent thread that discusses a similar issue with ICS links not working outside of a browser, even without any domain changes:

iCal URLs not working outside of a browser - Microsoft Q&A

In that thread, some users reported behavior similar to yours, which might help provide additional context or validation for what you're experiencing.

Since the issue only affects existing users, one workaround you might consider is:

Exporting the affected users' mailboxes to PST files,
Creating new user accounts,
Importing the PST files into the new mailboxes.

This approach could help reset any hidden configuration issues tied to the original accounts. However, this workaround is only viable if the users primarily use mailbox services and do not rely on other services tied to their accounts—unless those services can also be migrated to the new user objects. I recommend testing this with one user first to evaluate feasibility.

Additionally, I’d like to confirm: Are the affected users able to access and use their mailboxes normally otherwise? This could help narrow down whether the issue is isolated to ICS publishing or indicative of a broader mailbox configuration issue.

If none of the suggestions above resolve the issue, I highly recommend raising a support ticket directly with Microsoft via your admin portal. Their support agents have access to deeper diagnostic tools and tenant-level configurations that can help investigate and resolve this more thoroughly.

Wishing you success in resolving this issue, and please feel free to reach out if you have any further questions.

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

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YMO 15 Reputation points

2025-08-21T15:22:22.1766667+00:00

Hi @Kudos-Ng Thanks for your answer!

Yes affected users can use normally their mailboxes.
Kudos-Ng 15,050 Reputation points Microsoft External Staff Moderator

2025-08-22T13:39:36.7+00:00

Hi YMO,

Thank you for your response and for confirming that the affected users can still access and use their mailboxes normally.

It’s good to hear that everything seems to be functioning fine otherwise. Based on the troubleshooting steps you’ve already taken, it’s clear you’ve done extensive research and tried multiple solutions — yet the issue persists. I’ve also tried to look for further information in hopes of finding additional suggestions, but unfortunately, nothing conclusive has come up so far.

That said, it’s possible the issue may not be entirely on your side. I came across a few recent community threads where users reported similar problems with ICS URLs not working properly with third-party apps. One such thread is here: ICS Calendar Subscription Not Working – Has the Feature Been Paused? - Microsoft Q&A

From my perspective, although there’s no official confirmation from Microsoft, these discussions suggest that this could be a current limitation.

One suggestion I found is to download the .ics file locally and then try importing it manually into the calendar app, instead of using the URL. It’s worth testing to see if the behavior is the same.

Since I can’t confirm whether this is a limitation or a bug, I highly recommend raising a support ticket via your Microsoft 365 Admin Center. Their support agents can provide more accurate information and may be able to assist further with tenant-level configurations.

Wishing you success in resolving this issue, and feel free to reach out if you have any further questions or updates!
Kudos-Ng 15,050 Reputation points Microsoft External Staff Moderator

2025-08-25T08:14:32.8633333+00:00

Hi YMO,

Just checking in to see if you had a chance to review the previous response.

Your feedback would be very helpful in ensuring everything is on track.
Jack-Bu 6,770 Reputation points Microsoft External Staff Moderator

2025-08-27T14:10:08.2933333+00:00

Hello YMO,

It’s been a while since we last heard from you.

I wanted check if you’ve had a chance to review the latest response provided by Kudos. We hope it addressed your questions thoroughly.

If you have any further questions or need additional clarification, please feel free to return and ask at any time.

Additionally, if you feel this discussion has been helpful, please consider marking this thread as accepted by clicking “Accept the answer.” This helps surface the information provided for others encountering similar issues and contributes to building a stronger, more informed community.
Bogdan Pavel 15 Reputation points

2025-09-03T19:19:08.98+00:00

This issue is also being discussed in another thread and on external forums (e.g., Bugzilla ). It seems to be connected with how clients handle redirects for Office 365 shared calendar (ICS) links.

https://learn.microsoft.com/en-us/answers/questions/5529826/ics-calendar-subscription-not-working-has-the-feat

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ICS/HTML published calendar returns 500 error

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