Can't access custom checkout form sets anymore

Store URL: http://houston.skydivespaceland.com
WordPress version: 4.2.2
WP eCommerce version: 3.9.3 / 3.9.4
Gold Cart version:2.9.8
Theme: Divi (child)

I did this: Tried to access one of two custom checkout form sets to make a change to a field

I expected WPeCommerce to do this: List the contents of that checkout form set after I clicked the Filter button

Instead it did this: Went back to the homepage of Store Settings with a “settings saved” message. This occurred under 3.9.3; I took it as a vote to update to 3.9.4. Problem still existed. I deactivated all other plugins and switched to TwentyFifteen and flushed theme cache; problem still existed.

This problem could have existed for awhile and I wouldn’t have known it; I haven’t needed to edit the checkout forms in months.

I have three sites running very similar installations of WP/WPEC, and all three show this behavior. At least I’m consistent. 🙂 Thanks for any help!

https://wordpress.org/plugins/wp-e-commerce/