[Plugin: WP e-Commerce] Issue with uploading product images

A client of mine is having an issue uploading product images with WP e-Commerce. When uploading an image we’re getting an error that says: “Your session has expired. Please refresh the page and try again.” Clearing the cache and refreshing the page does nothing, and different browsers have the same issue. Uploading images under Media is not a problem, only adding product images under the product itself, so it’s not a permissions issue. We can upload the exact same image under Media without any problems – so it’s not a size or file format issue either. I’ve tried deactivating all other plugins besides WP e-Commerce, and it still happens. The issue is gone when we tried using the default Twenty Fourteen theme, so I assume the error is coming from the theme we’re using. We purchased the theme online and have customized it, although the original theme still produces the same error. I contacted the theme developer, and they said they believe the error is a conflict caused by the plugin itself since they don’t deal with the plugin directly and aren’t seeing any issues with the theme. We were able to upload images successfully a few months ago with this same setup – the issue only started happening recently. We haven’t tried uploading with the last few plugin updates, so I’m not sure when the issue started happening exactly, we only just noticed it. Any ideas on what it could be, or what I could look for? I appreciate any help or advice you could send my way!