Thank you!
I had to enable this function first, to see the lines you mentioned.
On the homepage it does not show anything like this.
All other pages do show these lines, except for one, the contact-page.
The homepage and the contact-page have one thing in common; both have a form which generate/fetch some user-data inside the HTML. This might very well be causing my issue.
The user-data is not being used, so if the plugin would allow me, I could disable it.
The plugin is PlanSo Forms Pro. I don't see any option to disable the user-data fetching.
Changing the plugins files is not a very feasible option, the plugin is updated quite regularly. Unregistering a function may help, but I don't know what instructs WP Super Cache not to cache these pages, or does WP Super Cache detect the user-data requests and decides not to cache these pages? The lines don't seem to be added using a separate function.
Thanks