This looks like a similar problem to SPIGOT-4576

Serialized ItemStacks with attributes lose their attributes in 1.16. Internal MC code prints an error "Unable to create attribute: null", meaning an NPE was thrown inside of nms.AttributeModifier.a(NBTTagCompound).

My best guess is that it's missing the minecraft: prefix on the Mojang attribute type, judging by the before and after shown in the attached screenshots (these attributes were created separately on each version, not deserialized, to see what they should look like)

Here is an example of an item serialized in older versions of Spigot that now throws this error:

item:
  ==: org.bukkit.inventory.ItemStack
  type: GOLD_SWORD
  damage: 7
  meta:
    ==: ItemMeta
    meta-type: UNSPECIFIC
    display-name: Emerald Sword
    Unbreakable: true
    internal: H4sIAAAAAAAAAONiYOBkEHIsKSnKTCotSfXNT8lMy0wtKuZiYGBgYmHgCA31dPHNLy75pWwn49jgkcrCwAkS8klNLC7ZIL3n3PTWNYvZGNgcc/NL80ocGCCAg4ElOCe/hIEjNzEzLyMxL4WDgRduhV9ibiqDcHpqXmpRZrJeYklJYnJ2cEFqagozA6d/QWpRYklmfh7UFLBaIdfC0syC3NS8EgWY8xiQXHbaPvRySoy/BbLLlkyfI7zYdut5hMsUiHWZCKrLXBJzE9NTSXEaAwCwmFFiUwEAAA==