Now I'm using Poser 11.2 instaed of 11.3 again. Version 11.2 runs stable with G2 figures. Poser Support told me Poser 11 has been discontinued since the release of Poser 12, so there will be no updates.

The other issue with the slowing down posers material room (shader editor) is caused by the windows 10 Update (KB4601319). I unisnatlled this specific Win10 update and stopped the windows update for some weeks. But this is no longterm solution.