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Thank you for your response. The problem of the non-generating of the thumbnail does it all the time with the same files. It never generates for those files and the files are in perfect condition, thy are not corrupt. I will send you a link with 3 files which one works with the thumbnail and the 2 other dont. 

You mentioned GhostScript that comes integrated with ImageMagick or is it a separate instance. Because in the Filerun, I dont see an option to put a path to the Ghostscript.