It looks like the mystery of the different stall speeds of two similar aircraft is solved. The difference was mainly caused be the pitot error. It turned out that the two airplanes had different pitot tubes. Though they looked almost the same from outside they were not identical. The dVpc in the speed range around the stall of one pitot tube dinverged much more than from the other. That was the reason for the differences in the stall speeds. After we changed the pitot tube the speed were almost identical - within +/- 1 kt.
Thanks again for all your advice.