Version restoration for layer does not work

[Ianunaut]

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(For a more detailed description of my issue, see my request over at the help forum: https://help.openstreetmap.org/questions/84421/umap-version-restoration-does-not-work )

I have recently added several dozens of objects to a layer of a new map and saved my progress. After revisiting the map some time later, the objects had disappeared. According to the version restoration function, the data is still saved:

However, the restoration of those previous versions does not work; the objects still neither show on the map nor in the object list.

Is there a way to make the restoration work or at least salvage the data? The map in question can be found here, "Projekte" being the problematic layer.