My analysis:
Two videos with badly assigned image files have the same "modification day" (explorer).
Therefore they both reference the same file in the column image file!
(something like C:\documents....Media Center 12\Cover Art\- 05.10.2005.jpg)
The mouse-over image is correct, if I remember rigth, also the thumbnails in the thumbnail view were correct, but when I start the videos I see that one image file is correct and the other one is not - presumably because they have the same image filename.
By the way, it does not help to remove cover art: the filename "-05.10.2005.jpg" is too unspecific.
After deleting one video from the lib and changing the date of this video by using a tool like the unix 'touch' I could reimport the video and now the image filename is different.
uff.
A touch without deleting the video did not help. The timestamp wasn't updated - possibly according to some of my customisation of MC12.
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Second question: can't I configure MC12 to reference to the cover file in my special subdirectory and not to copy the jpg. This would be a funny work around
By.