Page Permalinks not working on running site

Hey WordPress lovers!

Got a permalink problem on a running WP site. I’ve read everything I could find, but maybe/probably I’m missing something.

The problem
All posts and custom post-types have a nice post-name permalink structure and are working great. Except for the good old normal pages. Pages are not loaded with the post-name permalink structure. They only show up when I set the permalink structure to default.

I’ve added a test page in the backend from WP and the index.php, instead of the page.php template shows up.
This is the url: https://24uurcultuur.nl/test/ a test template file should load but the homepage, index.php is shown.

I tried (with no results)
– Creating page templates like page.php, page-test.php and page-5848.php, no result
– Resetting the permalink structure
– Checking the htaccess file, which seems okay
– When I set the permalinks to default, so with id in the url, the template shows up nicely.
– creating a now page, deleting the old + empty deleted items bin
– optimizing the database
– Setting debug mode to true, no visible errors
– caching to false
– disabling plugins
– activate a different theme

Anybody got some more ideas? Would really appreciate some help here ??

The best, Jonas