@Shaylois: Hi Shaylois, thanks for your input. I prefer to report such things as fast as possible so there is enough time to fix it or think about the suggestions. I won´t open a new thread if, for example, I find again some localization issues. It will be just added to the existing thread to prevent spamming the forums with threads of my own.
@Menorbriam: Wow, that was some quick fixing. Now my characters can equip all their shiny armour.
Unfortunately, now Gaulen does not benefit from the armour skill at all. His weight capacity is just derived from his strength and the skill bonus is not even in the tooltip. I started a new game and checked the custom party, the problem applies to them as well.