I tried uploading the same .zip file with butler and it worked fine this time. Possible bug on the browser upload functionality?
Unlikely it's a bug, since butler and web zip uploads are processed the same way. It's more likely butler zipped the file with a different directory structure than how you did it manually, which may have been enough for the file paths to match correctly. Did you check the browser console when it was failing? If you see a 403 error then you can look at the path of the file on that error to see how you're referencing the file incorrectly. Thanks