Hi there,
I checked and there are many javascript errors that are being triggered, I deactivated the ConvertPlus and its addon and the builder start working. But there are remaining errors originating from other plugins. Disabling all the plugins removes all that errors. You could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
And please update your ConvertPlus, the latest version has fixes for recent issues.
Thanks!