Ok, me again.
Problem "kind of persists".
Did a complete reinstall of 10.04 last evening and it went smoothly.
I could see Sarah for a short moment before the installer came up and the very moment the installer came up, away she was (could hear her still).
I finished the setup as usual (didn't have to see her anyway *g*) and came to the Orbiter in solid shiny black.
In fact all video output and the picture screensaver are affected but let me explain further.
During one startup, I happened to see the picture viewer for a short amount of time, therefore: it is rendering just fine.
So I jumped to console and killed OrbiterGL, picture viewer visible on xfwm4 workspace 3 (switched to it via mousewheel after enabling it via xfwm4-teak-settings).
So far so good, starting the orbiter up again (/usr/bin/screen -d -m -S OnScreen_Orbiter21 /usr/pluto/bin/Spawn_Device.sh 21 localhost LaunchOrbiter.sh) made the orbiter start up again and away went the picture viewer.
I thought it jumped to another workspace and therefore forced xfwm4 to have only one workspace (/root/.config/xfce4/xfconf/xfce-perchannel-xml/xfwm4.xml: <property name="workspace_count" type="int" value="1"/>).
After that, I restarted X via /usr/pluto/bin/Stop_X.sh && /usr/pluto/bin/Start_X.sh but the picture viewer is still hidden "behind" the Orbiter.
Therefore, somehow the OrbiterGL is not rendering transparent but in solid black where transparency should appear.
Could anyone provide me with details of how to debug this issue further?