I managed to repair my SD900
It took me some time to figure out what was wrong...
A litte plastic piece at a lens before the senor did break away. This little piece does interrupts a small light barrier to detect the lens position. Without this piece, camera did extend the lens like normal, than producted a mechincal noise (i think the noise came from the gears which move the lens before the sensor up and down) and displayed "LensRestart" Error and shutdown.
I glued the broken piece back to the lens and the camera does work again.
Fixed Lens before sensor:
The used lens i bought in ebay did seem to suffer from an other mechanical problem, so i used only the broken flex cable from it.