Hi @arzhadas,
This error looks most similar to an error reported in this thread. In that situation, the issue was caused by the texture format of the tileset. WebP images are not currently supported by Cesium for Unreal.
When you upload the dataset to ion, ensure that the “WebP Images” option in the upload setting is not checked. You may also need to change the export settings in the tool that you used to create the dataset originally.
If the tileset is not using WebP images at all, let me know.
-Alex