Andrew, are you using the Offload S3 plugin on your WordPress installation? I’ve just run into this issue myself and it turns out that ACF Image Crop doesn’t know how to talk to files that have been moved to S3 via Offload S3.
The solution seems to be using the ACF Image Crop addon for Offload S3, however that addon is only available if you’re at least on the Gold license of Offload S3. Bit rude if you ask me but that’s how they’ve got it set up.
Hope this helps.
