Ok, thank you very much
Kovid and
dwanthny for your answers.
The curious thing is that last time I bought prc ebooks, I used mobiDeDRM version 5 and did'nt have any problems to decrypt its and convert its to LRF
This time I used mobiDeDRM version 9 to decrypt these files.
Then what I'm going to do is trying to use again the old script to see if it works better.
Thank you again