Why does this pop up in my output when uploading an image to my game?

I have made an image with a transparent background and when I try putting it in my game I get this message in the output.

17:24:18.957 - Error while loading thumbnail for "Images/UPPERCUttTtT": HTTP 500 (HTTP/1.1 500 Asset is not approved for the requester)

Anybody know why this is or what I can do?

This is a web error. Something has gone wrong with the server, but there are no specifics to it.