Error: Updating failed. The response is not a valid JSON response

After installing the plugin, I set up a login page and used the basic shortcode. Ever since doing that, I see this error code whenever I try updating the login page.

I did some research, and it seems that there are many causes of this message. Some of them are related to the permalinks. I’ve tried changing them a few times and resaving. My problem is definitely related to this plugin. If I disable it, the error goes away. Has anyone else had this problem? The strange thing is that the page acutally updates.

But other than that, I love this plugin. I will probably be looking to get some of the extensions as I work on my site.

The page I need help with: [log in to see the link]