Moved here from direct support email:

Q. The notability app no longer can connect over CloudDav. I get a 404 error.

A. Did you recently upgrade the Notability App ?

Response: Yes, it was fine before this

A. We investigated, we have two internal staff using the older version of Notability and the newer one. The member of staff with the newer version of Notability also gets the 404 error so this appears to be an issue with their recent upgrade, you should contact their support.