In your specific case however, the Pulumi Service failed to complete the update on our end. (So your checkpoint was left in a state where it was referring to the older organization, hence the 404.)
Anyways, the reason why the Pulumi Service failed to edit your checkpoint file looks like it might be a bug in our underlying SDK.
Would you mind sending me your your stack’s checkpoint file? (Ideally at the version of the stack right just before you transferred ownership to a new location.) That would allow me to reproduce the error and find out what the problem is.
(It looks like it has something to do with the specific version of the Pulumi CLI that wrote the checkpoint, and the version of the code the Pulumi Service is using to read it.)