OK, solved. When you use the Photoshop actions to prep your TIFF's in readines for importing into Encore, the actions won't save your files as 8 bit TIFF's which is what Encore wants. If you start with 16 bit TIFF's, you'll be left with 16 bit TIFF's and Encore will spit them back at you without any useable explanation. The work around for this was to save my TIFF's as 8 bit first, then process as them as needed.
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Thanks Stan. Done.
I work with 16-bit Images too, and Scale to match my Project (am always doing this in PrPro, but it's the same). I always include a line in my Action to change the Bit-Depth to 8-bit, before doing the Save_As.
It would be the same, if I was working with CMYK Images (only do those for delivery to a printer), in that I would have a line in the Action to change Color Mode to RGB.
Glad that you found the issue and good luck,