ftp sessions fails with '421 Service not available' error.

Issue

• None of the virtual users are able to login though ftp using the  vsftpd packages provided with RHEL5.4, login fails with the following error.
• Updating vsftpd package in Red Hat Enterprise Linux 5 to vsftpd-2.0.5-16.el5 causes breakage in several FTP clients

For Example :

$ /usr/bin/ftp 192.168.0.1
Connected to 192.168.0.1 (192.168.0.1).
220 (vsFTPd 2.0.5)
Name (192.168.0.1:root): ftptest
331 Please specify the password.
Password:
421 Service not available, remote server has closed connection
Login failed.

• The issue does not happen if it's traced using strace.

Example:

# strace -p $pid_of_vsftpd

The problem is back as soon as the vsftpd process  detached from strace.