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Office 365 seems to be generating an invalid ics file?

Reverend Matt Gaventa 85 Reputation points
2025-09-04T04:18:07.17+00:00

My office subscribes to Microsoft 365, but my family are Mac users. I am trying to publish my Outlook calendar as an .ics file so that my family can subscribe in Apple Calendar.

I know how to use Outlook online to publish an .ics link, but every link I generate seems to be invalid. It does not pass ics validation at icalendar.org. I cannot successfully subscribe to it either using Google Calendar or Apple Calendar -- at gCal it just remains empty; on Apple I get an error 400. So it does seem to be something about the .ics link itself that is malformed.

I'm not the server-admin at my office, but I'd love wisdom about what may be happening or if there is anything I can try.

Outlook | Web | Outlook on the web for business | Calendar

Answer accepted by question author

  1. Jan Dzieciol 120 Reputation points
    2025-09-20T19:42:37.51+00:00

    Hello everyone, I've developed a small app to serve as a workaround until Microsoft releases a fix for the issue. Please note that it's still a work in progress, so use it with caution: https://calconverter.app/

    5 people found this answer helpful.

Answer accepted by question author

  1. David T 110 Reputation points
    2025-09-19T22:21:23.1233333+00:00

    I dug into and found out what’s really going on. The root cause isn’t the .ics file content itself, it’s how Microsoft is enforcing browser validation at the endpoint level.

    When calendar clients (like Google Calendar, Apple Calendar, etc.) try to subscribe, they don’t send a browser-style User-Agent. Microsoft sees that and returns an error page instead of the actual .ics file, saying the browser isn’t supported.

    The fix isn’t elegant, but it works: have your IT team set up a proxy that fetches the .ics file with spoofed HTTP headers to mimic a “real” browser (like Edge). Then have the proxy serve that file back to the calendar client.

    Basically, Microsoft didn’t account for non-browser access use cases in their client validation flow. It’s a silly oversight but unless someone inside Microsoft can escalate this to the Outlook Web App team, it’s up to users to duct-tape around it.

    3 people found this answer helpful.

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  1. Reverend Matt Gaventa 85 Reputation points
    2025-09-15T19:33:43.54+00:00

    Hi everybody.

    FYI that I spoke to somebody in Microsoft Tech Support today, and it was a highly discouraging call. They were able to test my .ics link within Outlook (i.e. subscribing to that link from some other Outlook instance), and it worked fine. But, of course Outlook can read its own files.

    The real problem is they would not listen to me that the same link does not work in any third-party calendar software, nor would they admit that it might be a server-side problem. All they wanted me to do was troubleshoot with Apple or Google.

    I left highly frustrated. Sometimes code is buggy. I get that. Tech support -- particularly for a business account like ours -- should be able to admit when a bug needs to be patched. Stop telling me the problem is in my imagination.

    2 people found this answer helpful.
  2. Thomas Tuck 5 Reputation points
    2025-09-15T06:42:49.6066667+00:00

    Still waiting for fix

    1 person found this answer helpful.
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  3. Nate 5 Reputation points
    2025-09-09T22:57:45.5366667+00:00

    Piling on to confirm the server-side nature of this issue. The ICS data is exposed over the internet and correct, but either throws 400 or gracefully does not load events when accessing through clients (e.g Google Calendar, Apple Calendar, or even Outlook itself).

    1 person found this answer helpful.

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