setlocale warning prevents password change

[sipo]

Hi,

When a user tries to change password  using the password pluging (poppassd protocol), the following error message is displayed: Could not set new password.

After debuging the password plugin, I found out that there is a warning message on the following line of ./plugins/password/drivers which prevents the password to be changed:
$result = $poppassd->readLine();

The message is:
sh: warning: setlocale: LC_ALL: cannot change locale (en_US): No such file or directory

However, the locale "en_US" is installed on the machine.

Your assistance will be very much appreciated.