1. checkout an item 2. display the checkout list 3. tick the check-in checkbox for the checkout 4. click "Check in selected items" button 5. error 6. refresh page 7. see that the item is checked in 8. repeat but do a renewal instead checkout: XHR, browser JS, server logs don't give any hint of an error happening, even less a cause. renewal: same but XHR response has "error: null" in it