Hi @Ruard_Veerman,
Sorry for the delay. We found that your tileset uses the KHR_techniques_webgl extension, which is not supported by Cesium for Unity. In fact, I believe it’s officially archived and shouldn’t be used for future assets.
There was a similar post with the same problem (B3dm files with texture data issue), but I’ll echo what @Marco13 said on that issue. It may be possible to update the tileset to no longer use this extension, but the details for the update process depend on the dataset. So we would need to look more deeply to determine how to best replace the extension.