Shuttle sent a stock, SKU or listing status update to Etsy, and Etsy answered with an error it did not explain.
Why it happens
A short-lived problem on Etsy's side. Etsy took the request but did not complete it. This is the most common cause and it clears on its own. Look for a newer row for the same product in the Sync History: if it shows success, nothing is wrong.
The listing's status could not be changed. Shuttle also reports this error when it tried to deactivate a sold-out listing, or reactivate one that is back in stock, and Etsy refused. Open the listing in Etsy and check that its status and quantities are what you expect.
A custom match update failed. When a product has custom matches, Shuttle updates the quantity of each matched listing one by one. If one of those updates fails, its row shows this error.
How to fix it
Wait a few minutes, then open Stock Syncer and scroll to Sync History. Click the search icon and choose the Errors & Warnings filter to find the row.
Send the stock again: open Products, click Update on the product and choose Update in Etsy. Changing the product's stock in Shopify also triggers a new sync.
If the row is under State Updates, open the listing in Etsy and set it active or inactive yourself, then check the next sync.
For a product with custom matches, open Products, open the product's More actions menu and click Sync quantities.
Still seeing it?
If the same product shows this error on every sync for more than a day, contact support with the product's name and the time of the failed sync.