I completed a couple of quests which rewarded items. I can not equip my hero with quest items, they are shown on hero's inventory management screen but they are grayed out and unclickable. I have required resources for these items.

You must be in the borders of your empire to change equipment. Maybe you are not?

and you'r hero need to be abble to use it, for exemple some hero can use crossbow and some other can't