[Membership 2 Pro] All pages gives a 404

Hi,
We want to hide some pages for non-subscribers. But when we activate the plugin all pages gives a not found message from the theme we use.
This happens for guests but also for subscribers who are logged in.
In the admin bar is the text "content protection is disabled" so pages should be visible..
Also disabled all other plugins but same problem.
Only solution was to deactivate Membership 2 Pro to get the pages back.
When we switch to the Twenty Nineteen theme there are no 404 messages, so it's the theme somehow. The theme we use (Nexus Themes) can create their own content besides the Wordpress content.
When visiting a page which contains f.e. a shortcode it is displayed but above it the message is displayed the page cannot be found.
Have there been other users with the same problem and can this be fixed in the plugin or should this be fixed in the theme? (I hope the first)
Gr, Age