Good news if anyone is interested....
I found the bug, the accompaning pdf documentation from Eurocontrol folowing the link in post #12 shows the correct equation.
=(-"ISADEV"/-0,0019812)*LN(1+(-0,0019812*"MDH")/(288,15+-0,0019812*"APAlt"))
the 1+ is prior the fraction... Sombody within ICAO seems not to be aware of maths basics ;-)
Like that, it works fine...
BG