Hallo,
ich habe inzwischen die Antwort auf meine Frage im Adobe Support gefunden
(sorry, dass ich da nicht zuerst geschaut habe):
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Error "Could not launch external editing application" When Using the Touchup Object Tool in Mac OS X
When you use the touchup object tool to edit a graphic object in a PDF file, Acrobat returns the error "Could not launch external editing application." The editing application (for example, Adobe Photoshop 7.0 or Adobe Illustrator 10) opens, but the changes you make to the object aren't automatically updated in the PDF file. To edit a graphic object in Acrobat when using Mac OS x, run Acrobat and the editing application in Classic mode, or open the graphic in the editing application, make the changes and save it, and then replace it in the PDF file using the Replace or Insert Pages commands.
This error doesn't occur in applications that open automatically in Classic mode, such as Photoshop 6.x or Illustrator 9.x.
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mfg
R. Schaefer