This error shows up in the Form Editor if you recently accessed the form builder app in a different ORG, in the same browser.
You get this error message if you have 123FormBuilder installed in both Production and Sandbox and you attempt to work in both ORGs in the same browser session.
Unlike Salesforce – which allows multiple sessions in the same browser, 123FormBuilder can only have one session per browser.
To solve this issue, clear the browser’s cache and cookies.
As a precaution, don’t access the 123FormBuilder app in two different ORGs on the same browser. If you need to do so, you could use two different browsers or an incognito window.