Are you using a two handed weapon? In any case, try removing your weapon before attempting to equip a shield.
Does it make a difference if you click on a shield in your inventory, or try to drag and drop a shield into the equipment window?
Have you tried equipping lower quality shields, with either low strength requirements or none at all?
Are you sure you still have 57 strength? Perhaps a spell, potion or food effect could be an issue.
Unless you want the bonus fairly early in the game, 10,000 gold rather than 1,000 would have gotten you 10 strength.
the Wishing Well (Spoiler).
