Not able to load/preview certain pages in designer

Summary

Some pages won't load in designer, although they seem to function when running the web application.

Content

Situation

We've built a web application in Visual Builder consisting of several flows and subflows, each containing a number of pages.

Problem

Three pages in our web application won't load in designer anymore. "Loading page" is shown for a long time, after which  "Page cannot be previewed" is shown with a "Reload" button beneath it.  Strangely enough, these pages do seem to load and function like before when running the web application using the 'Play' button.

The pages also show the following errors: