after getting the o-ring from the drever lab for the adapter to the turbo pump to pump the second chamber i figured out that the installed CF2.75 valve is leaking.
So while pumping with an external pump everything is fine but after disconnecting the external pump the ion pump can"t handle the leak rate and the pressure increases to a level where the ion pump switches off.
The leak is so large that you can"t disconnect the external one and quickly close the open port with a blank one until the pressure reaches the limit for the ion pump.
Because we don"t have a spare valve i decided to close this leaking port and don"t use this valve for further pumping. Instead i build an adapter to the small valve for a tiny hose (<1/4") already installed.
By using this adapter the pump rate is tiny but i still have the chance to lower the pressure below the limit of the ion pump.
By now the pressure is low enough to switch on the ion pump. The current is less than 10mA by now and decreasing, so i think the pressure should be ok tomorrow. 

In parallel i started building the thermal insulation for that chamber.