Customisation Suite problems

I have found several questions posted here that are similar to this but none of them have received answers and it has been about 4 years, so I figured I'd give it a try.

I am using the Maximiser Customisation Suite in a .Net application to access some of my Maximiser information.

However I've run into a brick wall in the fundamentals.

I am using the MaHost object to try and connect to an address book. My code looks like this:

 
            _session = new MaHost();
            _targetDSN = targetDSN;
            
            //Initiate the connection with Maxmizer's Address Book
            _session.Init(MaName, "DSN=MAS_" + _targetDSN, 5, "", "");

            //Verify that a user has access rights before granting a connection handle. 
            //This method must be performed in order to obtain a connection handle.
            bool canConnect = _session.DBLoginCheck("", _userName, _password, out _loginStr);
            if (!canConnect)
            {
                 throw new ArgumentException("Invalid userID or password. Connection to maximizer book has failed.");
            } 

Straight out of the Customisation Suite manual. This code works for 2 of my address books, but is failing on the third and I have no idea why.

Basically it is telling me that my user name and password are incorrect. However I test them by logging into the address book through the maximiser application and all works fine. The DSN and address book name are both correct. The code works because I use it with other address books.

Not sure what could be wrong, I would really appreciate some input.