It's a known bug. You might want to use some editor to remove the "disable spellcasting" effect from the item. The item was imported from IWD1 and was not edited properly.
Pretty old thread, but it looks like this never got a response. I've got pretty much the same problem -- my barbarian can't use his special abilities while wearing this armor, even though the description lists it as "light." Not a huge deal, but I thought I'd throw this out there.
Not sure if this has to do with the Ease of Use mod or just with the "re-added" forgotten armour, but my cleric cannot cast spells wearing the black dragon scale bought from Tahvo in Wan Village, any ideas?
Tried taking everything but BDScale off and still can't cast spells, however when BDScale taken off I can, is it just a bug in the armour?