I was able to downgrade to the last release from 3 days ago. If you edit the booksonic docker and change the repository line to "linuxserver/booksonic:1.1.Release-ls46" no quotes. I'll hold it here and try and update later once we see a fix for the 404 error, as I too was getting the same issue.
Thanks for the tip. It took me a second to find the "tags" page on the dockerHub page, but once I did, I was able to use the information to downgrade.