This is the 1922nd most frequent German word.
auftreten
"Auftreten" means "to appear, occur, or perform."
In this sentence, 'auftreten' is used to denote the occurrence of something, specifically an issue or problem.
Das Problem trat unerwartet auf.
The problem occurred unexpectedly.
Here, 'auftreten' implies the action of performing or appearing in a public event.
Er tritt morgen als Sänger in der Show auf.
He performs as a singer in the show tomorrow.
In this instance, 'Auftreten' (noun form from 'auftreten') refers to someone's behavior or manner of presenting themselves.