Hi @mickael ohlen
Check if there is any difference between the target user and other users. Are they in the same organization? Are they all work accounts? Are the affected users not hitting any API rate limits or quotas?
Also, if this is just an occasional error, try implementing retry logic in the API call. Sometimes transient issues can cause 500 errors, and retrying the request after a short delay may help.
If the above suggestions don't work, then open a support ticket with Microsoft Support to get help from the API owner.
Hope this helps.
If the reply is helpful, please click Accept Answer and kindly upvote it. If you have additional questions about this answer, please click Comment.