From European Axis Signals Intelligence vol 4 - Army High Command Signal Intelligence Service , p177
91. Work on Czech Agents' Traffic - The Vauck section worked on Agents' traffic of the Czechoslovak Resistance Movement. Mettig believed that the breaking in 1942/43 of two links running to England made possible the arrest of British agents in Czechoslovakia. The greatest success was achieved by intercepting the wireless communications of the Czech Resistance Movement in London. This was the only case in which Mettig is certain that it was possible for the Vauck section to break into an agent network by purely cryptanalytic means, and this was largely through breaches of security on the part of the Czech chief. After the system had been broken, the book for enciphering was found and the key recovered. In September 1942, the Czechs were about to go over to a new system but were foolish enough to name in the old system the book to be used for enciphering in the new system. Contents of messages solved on this link were nearly always concerned with reports on the political situation and activities of the Czech Resistance Movement, and were so important that for a long time the W/T traffic was allowed to continue unhindered.
A similar (but not identical ) version is given by Wilhelm Fenner ,Chief at Division B of OKW/Chi (Cryptanalysis).
From Ticom I-200 ‘’Interrogation of Min Rat Wilhelm Fenner of OKW/Chi’’ ,p9:
During the war, they only read Czech agents/ traffic when the keys had been compromised. They had read about 20 messages in 1943-1944, which were on a Ceasar received from the SD through the Abwehr ; he could not remember any details of the system involved. He thought that one of the agents must have been captured, and that the network continued to use the compromised keys. They never intercepted any messages, but received copies and keys with the SD stamp. He remembered that one message had given a long list of the names and addresses of Czech agents. He did not know of any Czech diplomatic traffic intercepted during the war.