Complex sentence: U- 256 (U Flak 2) returned to her base at Brest on 17 November 1943, because the Flak conversions were not considered a success, U- 256 (U Flak 2) was converted back to her original configuration in the winter of 1943/1944.
Split into simpler sentences: U- 256 (U Flak 2) returned to her base at Brest on 17 November 1943. Because the Flak conversions were not considered a success, U- 256 (U Flak 2) was converted back to her original configuration in the winter of 1943/1944.