Perhaps you might try to stop dosing kalk and try other form of calcium.
Strangely, I was also battling with cyno once, but after putting a chevron in there, it was gone. I know it sounds silly or it was just sheer coincident that something else had actually happened in my tank.
Ever since the chevron went into my tank, it started picking on the rocks and nibbling the sand on the sand bed. No more cyno since...
Wonder anyone else have the same experience?