The problem was that the asset I was trying to delete is an ImageCollection! To delete it, one needs first to delete each image individually before deleting the ImageCollection.
To do so in the Java GUI:
- Click on the asset details
- Go to Images, delete each image (trash icon)
- Then use the Delete button
Alternatively, if one uses the earthengine command line, one needs to pass the recursive argument to rm:
earthengine rm --recursive collection_id
