I also have tested with default new popups (no edit / modification).
I would like to tested the oldest version, do you have any? I have checked on your SVN in WordPress still no luck: https://plugins.svn.wordpress.org/popup-by-supsystic/trunk/
Thank you for contacting us regarding your question.
To determine the problem – please provide a link to this popup.
Please update the plugin to the latest version. This error should not be now.
If you still have a problem – contact us through internal support.