I have installed the plugin and pasted some HTML code into the HTML Exit Pop-up editor and everything works as expected.

However, when I add a <textarea> field to the HTML any subsequent use of the 'Create Opt-in Forms' tab is not possible - the screen is completely blank except for the WordPress admin sidebar, the 'Thank you for creating with WordPress.' text, the blue submit button and the special offer.

The other tabs are still available.

Deleting the HTML from the database makes the screen accessible again, until any content with <textarea> is added again.

Thank you.