Hi Urs,
I've solved my problem and I would like to share how I did.
According to various forums, the issue was related to the new menu configuration of P3D V3.4. During the update, the "content" did not fully install and therefore, several programs (GSX and Simplecam for me) menu were not showing up when asked.
The two solutions to these issues are :
- install the content of the version 3.4.9 (for most people, it apparently worked, but I had to go further and make a clean reinstall)
or
- a clean reinstall of P3D (as explaned here http://www.prepar3d.com/forum/viewtopic.php?t=120904 )
At least, that's what worked for me.
Hope it'll help somebody in the future.
Regards, Sébastien.