Question : 425: cannot open data connexion

When one of my client tries accessing my FTP he gets an error 425:Cannot open data connection. Why does this happends while All my other clients dont have any problem regarding this.

Answer : 425: cannot open data connexion

Could be your firewall also.  FTP has two types of data connections, active and passive.  Most companies block active data connections.

Have them verify that their ftp client supports passive and have them issue the passive command so that their client switches to that.  If they are using a GUI client, have them check their settings to make sure they are attempting to use passive.

Either way (active or passive) it is a firewall issue.  Your firewall obviously allows one of the two through, theirs does not.