Hi @Andreas
Welcome to our forum!
Suddenly we have noticed that it has stopped working the last weeks, any known changes from Microsoft regarding this ?
You can follow up on the known issues and fixes for shared calendars in this official document.
Known issues with Outlook Desktop Shared Calendar Improvements
In addition, I have the following details to confirm with you for your question:
What is the type of account that shared his calendar with you? Is it a POP, IMAP, Exchange account?
What is your Outlook client version? Please share with me the Outlook version that you are using as well. (File > Office account > About Outlook).
Please try to check if there is the same problem when you log in to Outlook on the web. If not, also check to see if any changes have been made to the shared user contact, and then try to re-share to check if there are any discrepancies.
Calendar sharing in Microsoft 365.
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