I haven´t installed the 64-bit geoprocessing, so I uninstalled 64-bit Archydro. Then I reinstlled Arcgis and Archydro (not 64-bit) but the Problem is still the same. I regocnized that not all tools lead to this error. For example the "Flow Direction" and the "Flow Accumulation" are working. Don´t know if it makes a difference.
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