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.
It seems that both were referring to a telegram sent on July 24 1942 by Leland B. Harrison, US ambassador to Switzerland, to assistant secretary Gardiner Howland Shaw (who was in charge of the State Departments cipher unit) warning him that an Italian official had met with Harold Tittmann (US representative to the Vatican) and had told him that the US diplomatic code used by the embassy in Egypt was compromised. The Germans obviously solved this message and thus attributed the end of the Fellers telegrams to Italian treachery.
However looking at the dates it’s clear that this was not true. Fellers changed his cryptosystem in June 1942, while this telegram was sent in July. On 29 June 1942 the Fellers leak was finally plugged and the Germans lost their good source.
Was Fellers to blame for this whole episode? The answer is no. He was not responsible for the security of the MI code. His mission was simply to report back to Washington using the officially designated code. It was not his mistake that he was not provided with a secure system.
In TICOM I-201 it is mentioned that messages from Baghdad and Teheran were solved.
Also according to ‘Swedish signals intelligence’ the messages of the US attaché in Stockholm were read by the Germans in 1942.