what *seems* to work, for now, is saving the swift framework options (just in case), then open an existing page, delete everything in the text box, and then enable the page builder. When the box is empty the page builder loads.
Then just close this page, and load another one. The page builder then loads as usual (for now at least).
It’s very strange because we have been working on this site for a month without problems, we haven’t installed any new plugins or anything in particular, and then in the last few days this starts happening
edit: yes thanks we will give you credential when it happens again