Image transforms not working properly
If I remember correctly, you will see urls similar to this http:\/\/mysite.dev\/cpresources\/404?x=CjvOQxsnA if you have generateTransformsBeforePageLoad as false.

It could genuinely be the case that the original file is not there so it's 404ing (as the link suggests it is).
If you are running multi-environments, have you got all the assets on your locale env?

Maybe try clear your cache, or, your storage/runtime folder if you have all the assets and try again?