This problem seems to occur with certain older themes. It is likely a bug in the form validation loaded with the theme. However, it could be a conflict with another plugin that loads older form validation scripts. It is always worth checking for conflicts with other plugins and themes. Try these troubleshooting steps to disable other WordPress plugins.
CabGrid Pro can also attempt to force its preferred form validation scripts to load. This is done via the advanced _settings.php file or via the free Defaults Add-on plugin. Try applying the following setting:
If the problem persists, please send us a message including the name and version of your theme, and your CabGrid Pro install URL.