If the builder (landing page builder) shows blank screen on the left, click CTRL+SHIFT-C to enable the browser debugger, and you may see this message:
error: No valid HTML template found
It was weird for me to see it, considering the identical installation on staging displayed no such error.
In any case. Re-installing fixed the issue, and even though other people report this bug, I won’t delve too much on it. If you’re encountering the same, just reinstall Mautic and it should be fixed. That’s the least time-consuming fix.