Credit for ID goes to Joern (see comments below).
This bird had been previously identified as Male Wood Duck x Tufted Duck hybrid by Pieter C. Brouwer.
Dear Ingo,
this is clearly a wood duck x Aythya hybrid, but the greyish vermiculated scapulars suggest that this is Wood Duck x Common Pochard and not Wood duck x Tufted Duck as suggested before.
Wood duck x Tufted duck have very dark scapulars, nearly black, in my experience; and show awhitish to yellow Iris.