Hello Richard,
I did a testing based on your error. I found that
the "Invalid Login" could be because of the virtual domain(s) is not in vqadmin.
Try logon to admin-toaster http://<your-qmail-ip>/admin-toaster/
login with your username & password. Once you
logon, at the main menu, Click on Add and
edit Virtual domains vqadmin-2.3.4
then click on List Domains, type in your domain,
see whether is it listed in the database.
Let us know the result.
Gab