From what I found it looks like it has to do with a PHP Deprecated warning:
Methods with the same name as their class will not be constructors in a future version of PHP
load_auth_class() returns that the class auth_ldap exists but it fails to initialize
so that the next call to $auth->checkPass() fails and the password check always fails
This prevents using ONA on a 18.04 Ubuntu server for example
I am closing this issue as I believe it to now be fixed in the development branch.. I'm currently doing testing on ubuntu 19.10 and have resolved these sorts of issues.
From what I found it looks like it has to do with a PHP Deprecated warning: Methods with the same name as their class will not be constructors in a future version of PHP
load_auth_class() returns that the class auth_ldap exists but it fails to initialize so that the next call to $auth->checkPass() fails and the password check always fails
This prevents using ONA on a 18.04 Ubuntu server for example
I'm affraid I won't find the solution myself