ADUS will not sync with a AuthAnvil server that is not joined to a domain.
Due to a bug in the ADUS web service, the AuthAnvil server must be joined to a domain for ADUS to function correctly.
The AuthAnvil server must be joined to a domain. It does not need to be on a domain controller, or the same domain as the ADUS client, it just needs to be joined to a domain.
Download the attached Hotfix and run the ADUSSetup.msi file on the ADUS server.
AuthAnvil 2FA v4.0 - v4.6
This issue was resolved in 2FA v5.0