Hello
I am have exactly the same problem. I am using a publish software called xsitepro and things worked fine to start with sites could be published and updated without any problem over the last several days. The software using it's own internal ftp program and today when i wanted to publish again to my live sites i got the same message
421 50 users (the maximum) are already logged in, sorry.
i doubt therefore that it is your software. I would also point out that i could access the host's cpanel manually, so i cannot see this is an issue of too many people loggiing onto the host at once.
Made a post elsewhere and it has apprently been escalated.
let me know of your outcome
clayton
sites
claytonpalmer.com
goals-and-solutions.info