Not sure if this is a domain mapping problem or something else. I moved a WP site into my multisite a while back and getting some 404 on a lot of images.
Here's the mapped domain:
And the subdomain on my multisite:
So -- images using the subdomain in the path work, but images linked to the mapped domain don't.
In other words, this image shows up:
But this image does not:
Shouldn't the domain mapping plugin be able to "find" where the file is located? I have confirmed that it is indeed on the server.