When I activate the entry "Menu/Vue/Aller
It's because you're trying to use a single key which is already in use, identified by the underlined letter in the menu label, e.g. '_Previous message'.
You are right about the behaviour in the menu. About the behaviour when modifying ~/.claws-mail/menurc, I do not know what had happened, because now, trying it again for the 10th time, it... just works fine... Thanks for the quick answer.