@Javier
You found the solution.
@mpg732
My problem was only on the terRemote side. FastStone Image Viewer and DevalVR are both PC programs. There was no problem in getting back to FSViewer but in getting back to the FS-Viewer-layout from where I started DevalVR.
The setup is:
1a) FSViewer thumbnail-view (layout FSV_1). Here a certain image-thumbnail is selected and I can hit a "fullscreen" button on the layout, so that FSViewer shows this image in fullscreen mode. Layout is changed to layout FSV_2, where I have a touchpad for panning the image after zooming in. Hitting the "ESC"-Button in this layout closes fullscreen and brings me back to layout FSV_1
or
1b) (again from thumbnail view and layout FSV_1) I can hit a button called "DevalVR" that starts the PC application DevalVR. The image then is also shown in fullscreen and the layout "DevalVR" is activated. When I hit the "ESC"-Button in layout "DevalVR" the PC-application DevalVR is terminated and I want to return to layout FSV_1 from where I came.
2) After switching to FSViewer's fullscreen mode (see 1a)) I'm in layout FSV_2 where I can also hit a "DevalVR"-Button. The effect is described in 1b) but if hit the "ESC"-Button in layout "DevalVR" now, I want to return to layout FSV_2 instead of FSV_1 like in 1b).
I hope this clarifies my setup. If you wonder why I want to have these different fullscreen options: Especially when watching panoramas, DevalVR is a lot more fun because it applies perspective correction and wraps around 360-degrees-panoramas seamlessly. Anyway, I'm lucky to have a solution now.
Best wishes
Tryanderror