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.