Error message "Failed importing documents" when publishing book to responsive HTML5

Hi,

I have FM12, version 12.0.3. This is a relatively big book (500 pages, conditional text, around 800 images). I have plenty of hard disk space and memory. All files are opened.  Still, 9 times out of 10  (approximately) I have a "failed importing documents" message after a third of the book was processed (not always happening on exactly the same file). Note that the conversion is terribly slow (28 min; around twice as long as when using TCS and Robohelp 11).

My guess is that the guilty ones are the images because they are unfortunately embedded (legacy reasons). With TCS5, Robohelp would crash for the same reason. I had found a script to externalize the graphics (ExtendScript of the week: Externalizing Graphics «  TechComm Central by Adobe); but somehow, this script is not working since I updated from 12.0.2 to 12.0.3. This means I cannot test my hypothesis. Although I wrote a few simple scripts, this one is quite complex and would be difficult to debug. Unless anyone here knows what could have changed to make the script useless after such a small update?

I mention the following just in case it's related and may help you: In the publish pod, when I open the Edit window of the Responsive HTML5, and click on style mapping, after a few minutes this message displays :

*Filename* could not be processed

In which of course Filename is one of the files in the book. Again, I don't know why this error happens.

Any idea about these problems?

Thanks!