So, back again, this issue is still there with N5.5.5
I tried to pickup old Defaults files like I did last year. Unfortunately it didn’t solve the problem this time. Erasing the Default file and launching Nuendo makes Surround Panner is instantly available. But I’m just loosing the whole default setup indeed 
Let’s sum up : After a while, pan windows can’t be opened. Double-clicking on track’s pan should open pan window (stereo or SurroundPan or SurroundPannerV5). It doesn’t any more. Having double clicked on tracks pan and opening “windows” menu, relative track’s panner is displayed on opened windows list then, but it’s actually not there. When minimizing all windows in windows menu, minimized panner can be seen with other minimized windows. Maximizing then makes pan window disappears.
The only way top solve this has been to replace Default file by an early one. Unfortunately I couldn’t make it work anymore, last solution is to erase Default file and to setup all preferences again…
Did anyone experience this?