I just tried this on the latest version of Hotcakes since you didn't mention which version you're on right now. Currently, the most recent release is 03.01.00. I'm not able to reproduce the error you've described.
This kind of thing generally occurs because of one of the following:
1 - Conflicts caused by the theme.
2 - Conflicts caused other modules loaded on the page.
In either case, the nature of the error you're seeing is the same. In order to determine the cause, I'd first suggest changing the theme the module is one and try editing the module settings again to add products. If that has no impact, restore the theme setting and try again on a page where this is the only module on the page.