But what's strange: atm, I connected the xbox through rgb-scart on the AV1-scart plug of the TV. On the remote, I have to choose "V-tape 1" (pressing the "AV"-button on the remote does nothing and yes, I got both AV plugs activated in the menues).
Now, I can well press "menu", and that's getting my to the "v-tape" menue, where I can change the sound and video settings.
But I cannot enter service mode by entering the "secret" code, I have to go to the "tv"-mode to do this - where I got only white noise, as I haben't got a tv signal coming in.
Back in "v-tape" mode, when I enter the picture menue, I can set the saturation, color and contrast, but it seems that I cannot save the settings. When the menu is displayed, the changes of the image settings are well shown, but when I press "go" to save them, the menu either stays on (with the changed settings). When I press "go" when on the menu item "v-tape", the menu disappears - but so do all my manual calibrations and the image is back to standard. The same happens when I press "exit": menu dissappears, and so do my changes of colours, saturation and brightness...

And plugging the box in the AV2 plug nets me the same results.
Ans ideas how to save my settings and how to enter service menu for the "v-tape" (xbox) mode?
If need shall be, I can take pics of all mentioned menues and sertings.
Thanks a lot guys for your help!
Actually... I like the mx4000 quite much, and it seems it has quite some potential. Hope to be able to use all this potential to the fullest.