Hi Guido,
Looking at the tileset.json it appears that tileset was intended to have a folder structure with it. When it was uploaded to ion as multiple files it lost that folder structure.
To preserve the folder structure when uploading to Cesium ion, you can create a single zip file and upload that to Cesium ion. ion will unzip the file prior to processing it.
Please let us know if that solves your issue. Regards,