After installing the "Army Shop" mod, which for some reason also installs the "Milky Cow" mod, this bug occurs.

In the starting location, "Nordling Island," there is a magic wall. When you activate it, a dragon's tooth should appear in your inventory. In the main quest, an entry appears: "Show the strange tooth to Runolv." But the tooth doesn't appear in the inventory! I go to Runolv, and there's an open quest marker above his house, but there's no dialogue line about the tooth, and the tooth itself isn't in the inventory.

Without the mod, the bug doesn't occur, of course. I suspect it's the "Milky Cow" mod; maybe it should be removed from the "Army Shop," especially since it wasn't originally there.