The greatest success for the German side was reading the messages of the US military attaché in Cairo. These provided Rommel with excellent intelligence and definitely contributed to his military operations.
Flicke has a different view of how the Fellers affair was betrayed. In ‘War Secrets in the Ether’ he says:
Then the miracle occurred. No, it was no miracle, it was a tragicomedy. It was so comical, so idiotically funny, that it had the effect of a passage from a dime novel. Or it was like a bad joke.
Also according to ‘Swedish signals intelligence’ the messages of the US attaché in Stockholm were read by the Germans in 1942.