Oh! I think I know
The instructions for installing the cluster (Installation for ViciBox Redux with Clustering) have a small mistake in them...
After the "Web Server Installation" portion, step 7 tells you "When you are done, you should be able to navigate a web browser to "//server/vicidial/admin.php"
Well... yes, you can navigate there, but you will not be able to login
Reason being that it's the Telephony Server installation that is the one that runs whichever script it is that inserts the users table into the DB. I don't know why, but the DB installation alone does not run it.
Once you install your telephony server with the proper credentials to connect to your DB, you should be fine.