I have never set a defaultAccessToken for Cesium.Ion and I am not using any custom imagery providers. The map tiles have worked for a long time and I am just now seeing this issue within the last month or so.
I know that I can create a Cesium ION account and create an access token, but I am wondering why the default access token no longer works for me.
This is because the default access token that ships with CesiumJS is not meant to be used in production. It’s just there to make it easier to get started with Cesium. The default tokens get revoked with each new release to prevent using server resources without paying for them.