This problem almost killed me the other day. I upgraded a Filezilla FTP Server to the latest version. I went ahead and made a backup of the FileZilla Server.xml file and ran the installer. An hour later I get an email from a user informing me that they could not longer log into the server. After logging in myself I saw that the server was refusing connection. My heart sunk. I tried reinstalling, fresh config files, everything, I changed every connection related setting in Filezilla to no avail. What did work was I manually copied the user and group configuration section out of FileZilla Server.xml and copied them onto a fresh config file. The server started accepting connections again. This means that one legacy setting in the old version of Filezilla Server must have been preventing new inbound connections. I had to reconfigure the settings of the FTP server but at least the user/group settings were still working. Crisis averted, but barely.