Well, this is really intriguing.
I´ve copied a save game from the new pc to the old one, and the bug is still there
... Then I´ve begun a game in the old pc to copy the save game to the new one, just before a battle, but
!, the bug is also now in the old pc, where yesterday it wasn´t.
So I´m afraid that we are talking about a infectious bug.
I´d like to upload the file but I don´t know just now how to do it, so if someone, STARDOCK specially, is interested in to take a look at it, with the risks that are involved, I´d be grateful for following his advices on how to upload the file.