JSON Error
This error is the result of mixed sessions in the browser.
A mixed browser session occurs, when switching from the Admin side to the Cart side in the same browser, within the same session.
Simply logout from and close all the Configio tabs on the browser, clear the cache and cookies, then try again.
As long as the browser is only accessing one side, or the other, either only the Admin side, or only the Cart side, this error should not occur.
NOTE: Impersonation of an account as an Administrator, does not have this issue.