Just joined to domain and still having same issue. I even limited my own account to just bookstack and it wouldn't let me log in. Error says "These credentials do not match our records". As soon as I remove Log On To restrictions, these accounts can sign in.
Verified domain join by running
id DOMAIN\\user and ensured I was getting user properties back.
Followed these steps to join to domain... https://www.server-world.info/en/note?os=Ubuntu_18.04&p=realmd
Server gets IP from DHCP, there's a new computer entry in AD further verifying it successfully joined domain, and I also verified the DNS entry for the server. Not having passed AD authentication before for a scenario like this, not sure if this is typical or some issue with bookstack authentication specifically.