On our website 3d secure is enable for the Braintree payment method when customers do click on the place order popup will open but when the customer cancels the button it just closes the popup and the loader is showing continuously.
When one-step checkout is enabled then it is not working and when disable popup closes and it shows a message which is I think is correct.
Please provide us a guide to fix it.
For this error, please edit the file app/code/Bss/OneStepCheckout/view/frontend/web/js/view/place-order-btn.js as below to fix:
If you have any other questions, please feel free to contact us. We'd be happy to support!