BuddyPress.org

edit: I found the error.
It is indeed a translation error.
I found the translation of Buddypress in wp-content/languages.
With PoEdit I saw that “%s wrote a new post” was translated as “%1$s heeft een nieuw bericht %2$s geschreven”.
I changed the translation to “%s heeft een nieuw bericht geschreven”.
PoEdit generated a new mo-file which I uploaded to wp-content/language. Now the problem is solved (for me).