Hi lennart
You need to delete the Preview Cache.
It is located in the same folder as the Lightroom Catalog. The preview cache name ends with Previews.LRDATA and should be deleted. Do not confuse this with *.LRCAT, which should not be deleted.Below is the image of what it is named as Default LightroomcatalogPreviews.Lrdata.
Location /Pictures/Lightroom on Mac and My Pictures\Lightroom on Windows.
Delete this Folder and restart Lightroom
Let us know if that helps
~Akash