Hi Puybaret,

Yes I have two screen.

I tried to deactivate one screen and then start sweethome3d, then I was able to detach and attach the 3D view. Also after restarting the second screen.

I then started Sweethome3D again with two screens, there was the error again.

So it is important if two screens are active when starting Sweethome3D

Cheers Brill