for the record:
I had my guisettings.xml full of entries like this one
Code
<resolution>
<description>3840x2160 @ 50.000000 Hz</description>
<subtitles>1080</subtitles>
<pixelratio>1.000000</pixelratio>
<overscan>
<left>0</left>
<top>0</top>
<right>1920</right>
<bottom>1080</bottom>
</overscan>
</resolution>and that was it what caused the trouble. No idea how they came in ...
anyway, now it's resolved !
