Hi @PawelJarosz-0356,
Did this issue occur on this mailbox that was converted from a shared mailbox only?
If so, according to the Official documention, the feature of Insight is available to a valid mailbox(a user mailbox from the start ), for more information, please refer to the section of "Why don't I see the Outlook Insights add-in?" this document: Insights Outlook add-in
In this case, it's recommended to create a new user mailbox for your need.
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