Hi Yleclerc- [O365],
Since the issue has been resolved by resetting folder name , it’s hard to find out the root cause. Many factors could cause the issue. For example, user did some operation such as change the language outlook Web App or others. Or this could be a temporary issue.
The fix doesn’t require a high level of technical. You can share the method to your user.
For the time being, I suggest you keep monitoring it. If the issue happens again, feel free to post back. Thank you for your understanding.
Regards,
Amanda