Vlad I know my error now. As you said, the correct and working lens is "Fediverse Forums" - it works. And I previously used maybe an old and outdated "Lemmy & Kbin" lens, which seems not to work properly.
I believe it is pretty confusing to have both lenses. Maybe the old one ("Lemmy & Kbin") should be removed from the lens list. Also, in the last changelog (which links to this post), the "Fediverse Forums" lens is referred to as
Lemmy/Kbin search lens
Although it is technically correct, I believe it was also confusing when the other similar non-working lens exists.
Does that lens exist only for me?
It is also not possible for me to edit or remove that lens or deactivate it.
It returns to the active list every time I remove it from the list (green toggle).