I've found that when I create a "root ftp user" for a domain (basically, a user that can access /var/www/virtual/domain.tld/), after a couple days, I can no longer use it.
Suddenly, ajaxplorer refuses to acknowledge that it has a "repository" assigned to it.
Re-making the account does nothing, and I suspect that it does something to the username.
This has happened with two of my accounts and I can't keep changing the name of the admin account!! I have other people who use that specific FTP account as well >_>