We are having some issues here regarding both Upfront Editor and Builder, and how they behave differently depending on what option we select for 'Permalinks.'
When 'Plain' permalinks is selected, Editor behaves with full functionality (i.e. photos are more easily manipulated), while when 'Post name' permalinks is selected, my options behave differently and in the case of inserting media (photos for eg.), the entire interface becomes blurred and I am unable to proceed with selecting any sort of media.
The only way we can access Upfront Builder appears to be when the same permalink option above (Plain) is selected. When we have 'Post name' links enabled, hitting 'Edit in Builder' on any theme gives us a 404 page.
I get the following warnings when selecting 'Plain:' "Upfront Exporter requires Pretty Permalinks to work. Please enable them here;" and "Upfront requires Pretty Permalinks to work. Please enable them here." The behavior I'm seeing is the opposite, however - Upfront works only when Pretty Permalinks are off.
There's another factor as well. When plain permalinks is selected and we appear to be able to edit the theme in Builder, it appears to save correctly, but we do not see any changes on the published site. Only changes made in Editor appear to affect the actual site.
At this point, we have not settled on a particular theme, nor have we determined that we even wish to create a new theme using Upfront Builder. We have simply been experimenting for the moment.
I'm working within a university IT structure, so we have a somewhat non-standard Wordpress installation, but it works well with the other Wordpress sites we host. I can get more detail about the configuration (eg the .htaccess file) if I can take my IT folks on this end a good reason why it might be involved and what change might be helpful.