The “Header value contains invalid characters” is likely caused by Cesium for Unity trying to put certain information in the request headers when it communicates with Cesium ion, and in your case that information contains characters outside the ASCII character set. I’ve written a bug for this:
Until it is fixed, you can likely work around it by renaming your project to use only ASCII characters. I understand that’s not a great long-term solution, but it should get you running for now.
Kevin