In the first years of the war in the East the Germans could read practically all the Soviet codes. In the period 1943-45 however the SU upgraded its cryptologic security. The top level 5-figure code was enciphered almost exclusively with one time pad and the insecure 4-figure codes of the OKK type were replaced with SUV tables.
This meant that the work of the Germans codebreakers became much harder. However they were helped in their work by a serious error in the Soviet Union’s radio security. Special units controlled by the Soviet High Command (assault, engineers, artillery, supply) did not follow the strict protocols of the standard military formations nor did they use secure codes. These errors allowed the Germans to circumvent the new Soviet procedures.
By monitoring the traffic of the GHQ units assigned to large Soviet formations their concentrations and movements could be followed.
Source: FMS P-038 ‘German radio Intelligence’