The following error appears when exporting imp file to PDF:
ERROR: Can't break up a colored image in its colorants yet
INFO: No PDF output file is written
ERROR: Exception of class imp_ethrowable: Error while generating pdf file: testesko_plate_bema_Trimboxes-Stalen-Basis_200_W1_F.pdf
The error appears for only a few .imp files. Other files process correctly. And when you launch the problem file directly into a standalone Proof Imposition task (identical settings) it goes through without any problem.
This problem is solved in Automation Engine 12.1.1 (released in September 2013).