Well, I think I got it..
My script has about 170 lines of code. Eight parameters are needed.
it can look redundant, but I inserted my script into a Model. so, I had to set the eight parameters again. Model --> Properties --> Parameters.
Everything was working nicely. But, then, once, I noticed that the correlative order of my parameters into my Model had changed. in detail, the third one was now at the end of the parameter list, in eight place. That´s why things could not be working at all. Surprisingly, every time i corrected, when I opened the windows again and again, the order changed again by itself.
After a while, it worked!, the parameters looked to be in place. But the Model did not run..
Now, I have a new model (from scratch) and it is working.
Perhaps, there may be an inconsistency between the script parameters and model parameters. Overall, ESRI should have a look at that because many times I had to re-start ArcMap, too many crashes.
Jose.