This can be either a filesystem/folder permissions issue or a vsftpd configuration change, due to a vsftpd version change.
Start giving us the vsftpd version which is installed and running, issuing the command
at the command line after ssh/telnet the box as the 'root' user, using the same password as the webUI.
Please post the output of the above command, even if you solve the issue by some other method.
Do you have a non empty /etc/vsftpd.user_list file? Use the command
cat /etc/vsftpd.user_list
to see its contents, if any.
Can you save/put files on the server, either as an anonymous or a normal user?
The attached file is the standard RC4.1 /etc/vsftpd.conf file, what is your file current contents? use
to see and post its contents. You can afterwards replace it with the posted one.