Rick, thank you for the answer,
my preferred candidate for the problem is not the access rights, but rather the size of the table. The tracker currently has about 3900 items.
Supposed an item that is sometimes visible does not appear in the tracker calendar for a specific user. If access privileges were the cause, the user probably won't be able to see exactly this item if he accesses the tracker directly, but there the item is visible to him.
Supposed exactly this item does not appear in the tracker calendar. Then the user should not be able to see it, if the tracker calendar hides by a filter some items which were previously visible to him. But now, when only a few items are shown in the view, the missing one is suddenly visible.
The miracle is, if I restrict the number of visible items, some items become visible.
I do some additional tests, especially concerning the tracker size, and report a bug, when I know more details