I've been digging this bug and found that it affects only medium shields which description mentions "this shield have been used by a xaurip".
I'm not sure what is the problem though, because i can equip it in the inventory screen.
It seems to me that for some reason i shouldn't be able to equip it and that these shields are meant only to be sold ? Anyway that's the only explanation i could think of, and there is definetely something not ok here. Not game breaking of course, but still annoying.
I'm playing with version 3.02.
Also excuse my poor english, i'm french.