I'm seeing exactly the same issue with v5.5 on an ubuntu host. After creating and setting up a user db, then starting the calibre-server with authentication enabled, I'm getting the same error:
Code:
> calibre-server --userdb /tmp/users.sqlite --enable-auth --port=58083 /data/ebook
Failed to communicate with "/interface-data/init?1606339410018", with status: [403 (error)] Forbidden
The user me is not allowed to access any libraries on this server