The first impression was that the server was broken but after incrementing the timeout in the options of filezilla to one minute we were able to access the ftp, but this was painfully slow. After some troubleshooting we start looking for changes in the server, network, etc. and we discovered that the only thing that changed was an intranet DNS that we installed using Zeroshell to be able to resolve intern hostnames. The solution was disable the reverse DNS resolution in the proftp config file and add a couple of tunning about the number of files and folders that can be get at a time from the ftp server.
Add this lines to the end of the proftp conf file and everything starts working just perfectly.
ListOptions "" maxdepth 3
ListOptions "" maxdirs 10
ListOptions "" maxfiles 1000
Source of the solution: