Planning second Post fails everytime

Two of my wordpress websites encounter the exact same problem which started months ago out of the blue..

What happens is that when i make a Post and plan it for the next day (or any other day) everything is fine. But as soon as i make a new post and plan it for any other day the first post just gets replaced by the content of the second planned post. (They post number is exactly the same).

I turned of all plugins, got the newest WP version, turned of all cashe and varnish and tried two browsers. Still this bug keep persisting.

Weirdly enough when i swap browsers i can make a second planned post but then i got the same problem after that (so it does create a new post number, but only one)…