Calendar is missing a view which would present all activities. Now I can choose 'All planned work times' etc, but instead of separate views I want to see all data in a calendar, so for example I won't schedule someone a task during a leave.
To make all activities visible, click Calendar on the main menu, and then click the filter icon in the upper-left corner of the window. A filter panel will appear, where you need to select the selected activities in the Category filter in order to display them correctly.