Having the same issue today. Content Builder is totally missing after upgrade to Pro 1.2.3. No caching plugins are installed.
I went to edit a page by navigating to Pro > Launch. The attached screenshot is what I saw

Content build wont load if trying to access it any other way either. For example, from Pages > Edit with Pro or from the admin bar when the front end of the site is loaded.
I reverted to a backup I made just before the upgrade attempt so giving you login creds wont add to this discussion. Can you speak to the nature of this error and suggest a fix in general terms? What’s going on here?