I’m also having this trouble, using Cesium for Unreal. This happened once before, but was a transient error that we never got to the root cause of.
One thing I see but don’t understand is that the 401 error URL has an access token that differs from the project token. The upshot is that “Troubleshoot token” shows all-clear, but the imagery does not load. Does the Ion client use its own token for Bing?
Error token: eyJhb…1XvIk
Project token: eyJhb…ZlISo
n.b. Other imagery, e.g. Blue Marble works fine