Now it works, but don't know why, since the only thing I did has been:
1 erase the last article I've posted through WLW
2 paste the same exact html content in a new article with same title

On another pc, with another joomla account, WLW didn't allow to post new articles even after the first WLW worked correctly,
so I recreated an account with the same data, and now it can post...

Not exactly a reassuring behaviour, isn't it?

Please, let me know how I could mess with utf-8 if db and clients are all set on utf-8