I have already tried to deactivate all the plugins that are not bundled in the theme, but it doesn’t make any difference. Also I set the memory to 256mb.
Only when I have content in the swift page builder, it won’t load. Without content the edit-page will load without any problems.
It’s very odd, because when I press shift-F5 and refresh the edit-page (after it fails to load), the page builder loads again without any problem.