Hi,
There appears to have been a change to the API call to require the quantity along with the request to update the SKU.
We can look into making it so it is no longer required, but in the mean-time, if you also include the listing quantity AND the SKU, it will accept the update.
Ex, if your quantity was 1, just put 1 quantity, and "xyzwhatever" sku.
Tip: if you are updating a lot at once, you can set a formula to match the current quantity and drag it down so you dont have to type them all in.