I'm trying to upload a JPG 9927 px x 14043 px through editing a WooCommerce product, and adding a downloadable file. Every time I try, I Receive an error:

Post-processing of the image failed likely because the server is busy or does not have enough resources. Uploading a smaller image may help. Suggested maximum size is 2500 pixels.

There is plenty of disk space (20 GB).

I've CHMOD all directories from /wp-content onwards to 755, but the issue remains.

The PHP max_upload_size is 128MB, and the images are no more than 35MB in size.

Help appreciated.