ForkLimits?

Sometimes ftp-proxy (transparent version) logs me off the server. In the log i have found several messages: ftp-proxy [17506] <07/21-12:23:16> USER-ERR reject: '172.20.1.206' (ForkLimit 40) What it means? I didn`t set any limit and user from 206 was only scanning ports on our sever... lzap