>This morning, when I click on the button "New messages", I get weird behaviour. When clicking it several times, it will show me the same messages over and over again, and the count of new messages doesn't go back to zero, but stays around 30.

Yes, no forum in your setup is defined for auto expand. So, the way we did it at first, was that the related messages of those forums that do not expand would remain as new until the forum is expanded. But, as recommended since then, it seems it is better to have the count of new messages not affected by the new messages in forums that do not expand.

So, in your case, as you don't have a forum that auto expand at startup, the count will always be 0. The way this is now mostly used is that a member usually have at least one forum that auto expand at startup. So, the other forums are only used occasionnaly and collapsed at startup. So, the count of new messages would rely only on the expand forum new messages.