Error, the file permissions could not set

I am having difficulty uploading images to my gallery. I have the latest versions of WP, NextGen and NextGen Public Uploader installed. Although I can upload images directly from within NextGen itself, when I try to use the Public Uploader (using a short-code on a page) I get the message: “Error, the file permissions could not set.” (sic) The Folder “wp-content/gallery” has file permissions of 750. I’d be grateful for any assistance or advice.

https://www.ads-software.com/plugins/nextgen-public-uploader/