Answer

You can use a unique number for each thumbnail in the "sort order" option in each thumbnail widget. Please make sure not any two thumbnail widgets have the same sort-order number.
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When I add thumbnails, let's say in the design mode from left to right as this: nr.1, nr.2, nr.3, they are shown the opposite order like this: nr.3, nr.2, nr.1 in the browser preview. The option "show thumbnails randomly" is not activated.
Where is my mistake?
You can use a unique number for each thumbnail in the "sort order" option in each thumbnail widget. Please make sure not any two thumbnail widgets have the same sort-order number.