The option servertimezone offset seems to be ignored, but setting "Daylight saving time" from "Adjust remote timestamp to local convetions" to "preserve remote timestamp" solves the problem.
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the old version of WinSCP 3.x does preserve correctly the timestamps by "Keeping remote Directory up to date...". But the new 4.0 version does this not! It sets the timestamp to the wrong timesone (1h erliear), so that a make can't find changes...