40x errors are cached in an exponential back-off manner. If the transformed image is accessed before the original is loaded, the system returns 40x and this returned 40x error is cached, first for 1 minute, and the cache period doubles every time until settling on a maximum 24-hour cache period. If you upload the original after the transformed image is cached, you need to wait for the cache to expire for the transformed image to appear, or use a different transformation.
Other transformed images that were first accessed after the original was uploaded will work as expected. In other words, please make sure the original image is fully uploaded to your Cloudinary account before attempting to access any of its transformed image's URLs.