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.
Regarding the solution, that problem seems quite trivial.

The site that you want to save into your reading list doesn't disappear shortly after but is put on the bottom of that list.