Why does the website I'm putting into the reading list in Safari disappear shortly after?

When I put a website through "command/shift/D" into my reading list, that site disappears after a couple of seconds. It happens to every site I'm putting into that list and also tried to restart Safari or my MacBook Pro. But it was no use. How can I solve that problem? One thing to note, I've installed macOS High Sierra just this morning and that problem also appeared today. So, there might be a connection.