Unable to edit the website with version 2.5.0.1, only deactivating the plugin Speed Up - JavaScript To Footer.
This is for everyone who has had problems with this update and has Speed Up installed.
Forcing JS to the footer and deferring it will often break anything that relies on JS to load in the header like the builder UI
Ask the plugin author if there is a way to disable the plugin in certain situations, they often provide a filter for this.