I pre-approved an enquiry from a guest this morning and when she went to accept and finalise the booking tonight she also got the 404 error message. I tried sending a special offer then declining the request to free up the dates so she could try again but still same issue.
In the end she called customer support (beat me to it as next thing to try) and they said it is an issue with the website and to use the app. She was then able to make the booking. If your guests are still having trouble, call support and ask them to push the booking through if unable to access an app (rather than the website) to book.