Hi torsten, on certain video cards the older XQuartz version bundled with MoI seems to have difficulties.

Please try installing the latest XQuartz and changing the setting in MoI to use the system installed XQuartz rather than the bundled one.

There are some instructions here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6967.4

Please let me know if that works for you or not.

- Michael