Cannot Sync Audible Books to iPhone with iTunes

I am unable to sync any of my Audible audiobooks to my iPhone X. This has only started happening since the update to iTunes last weekend. Whenever I sync from iTunes to my phone, I get "Some of the Audible files in the iTunes library were not copied to the iPhone 'Stainlyss's iPhone' because you are not authorized to play them on this computer." No audiobooks are on my phone, now.

The files are authorized, however. Originally, I was stuck in an authorization loop where I could not play the books in iTunes, but after working with Audible's tech support, they now work. (I uninstalled and re-installed the Audible Download Manager and then downloaded a book from the Audible library to force authorization) So the files are definitely authorized. I have even de-authorized and re-authorized them from within iTunes, to be sure. So iTunes authorization is not the problem...but I cannot copy the files over to my phone.

Help?

Thanks!