Hi @spiemel, I am not aware of any limitation of the software that requires both screen to be enabled to allow eye tracking… however, I will try to reproduce the error you have mentioned and get back to you.
I assume you are disabling the laptop screen from within windows display properties? And that the eye tracker is calibrated for use on the external screen?
Did you try connecting the external monitor via regular DVI cable? (Worth testing out)
It might also be worth reinstalling and configuring the Tobii software with the Laptop screen disabled beforehand as a possible workaround whilst I investigate the root cause.