Random box in exported JPEG

OK, this is getting ridiculous. This version of Photoshop is costing me money.

Has anyone seen this before? In the middle of exporting a number of similar assets, a random box started showing up in the exported JPEGs. I went back to an earlier file which had exported correctly, made no changes, and attempted to re-export it; it has the same problem. 

Quit Photoshop. Restarted. No difference. 

Deadlines loom. Anyone have a clue what this is? I've tried turning layers off throughout the file, but no one specific layer seems to be causing it.

First image: correct, exported earlier. Second image: no changes made to layered file, exported minutes ago