I was having the same problem until I disabled W3TC. I was then able to save the data and after re-enabling W3TC everything was still there. So if you have W3TC active, disable it and save the date and turn it back on. If not you’re not using W3TC there is more than likely some other plugin conflicting with WP SEO Structured Data Schema. To the developer…Fantastic plugin you got here. Thanks for sharing it.