Hi,
I am also experiencing this problem. We’ve recently migrated our website to a new hos (from WP Engine to Kinsta) and with it, a new version of WordPress (4.9.9 to 5.0.3). I installed 3D Flipbook, and am getting the above error message when trying to modify any of our existing flipbooks, or creating new ones.
Please let me know what I can do to resolve this. Thank you!
