Thankyou to everyone for your responses
In /var/log/messages I don't see any errors for pure-ftpd
In /var/log/syslog I am seeing the following error for pure-ftpd:
pure-ftpd: (?@?) [ERROR] Sorry, invalid address given
I am connecting to the server via it's hostname (eg. domain.com.au) which is a valid ISPConfig site on this server (no problems on the web access). What is strange is it seems to be random. Sometimes I can connect fine, sometimes it says invalid address.
I have tested both active and passive transfer modes on the client software. I have also tried limiting the amount of simultaneous connections that FileZilla opens to just 1.
I don't really know how to make a test ftp session inside the server or on the local network... as far as I was aware I can only connect via FTP if I specify a valid domain name for the server address... I can't just connect to it's local IP address (even though I am on the same LAN) or I get nothing but the "Invalid address" error.
I have tested with many different domain names that are sites on this ISPConfig server and it is always the same: randomly getting invalid address errors