Hi @Florian Schrötter ,
Does this issue persist all the time?
Can you see the calendar show normally in desktop client?
Please make sure this user has assigned the Exchange online license in their tenant. That is required to have calendar tab.
Another reason it could just be hidden. You can customize your icons in teams at the bottom if you click the little line right above the icons or scroll up from an icon you’ll get more options to choose. The calendar may be here.
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