Known issue (and solution) for JavaScript Error and Security Vulnerability
Everyone that uses (or has used) server validation in CSS Form Builder should download the known issue in the Download Center - on the CSS Form Builder downloads page - as we discovered a security vulnerability. All you need to do is copy/paste a few files before you create any forms; there are also simple instructions to fix the issue easily if you have already created forms.
Also included in this known issue zip file is a fix for the JavaScript error you receive when applying Required / Not Blank / Selection Made client-side validation.
We are planning a dot release in the coming months that will include these fixes so that in the future, you will not have to take this extra step. You will receive an email / Memento announcement when the dot release is available.