Proftpd service dies

[Devon Harding]

About every 3 or 4 days, my ProFTPD service stops accepting
connections with the following message:

[root at mars proftpd]# ftp localhost
Connected to localhost (127.0.0.1).
220 FTP Server ready.
Name (localhost:root): jdoe
500  not understood
Login failed.
ftp>

I keep on having to restart the service to get it going.  Am I doing
something wrong? Here is my proftpd.conf:

[root at mars etc]# cat proftpd.conf
# This is the ProFTPD configuration file
# $Id: proftpd.conf,v 1.1 2004/02/26 17:54:30 thias Exp $
ServerName                      "ProFTPD server"
ServerIdent                     on "FTP Server ready."
ServerAdmin                     root at localhost
ServerType                      standalone
DefaultServer                   on
AccessGrantMsg                  "User %u logged in."
DeferWelcome                    off
DefaultRoot                     ~ !adm
AuthPAMConfig                   proftpd
AuthOrder                       mod_auth_pam.c* mod_auth_unix.c
IdentLookups                    off
UseReverseDNS                   on
Port                            21
Umask                           022
ListOptions                     "-a"
MaxConnectionRate               4
AllowRetrieveRestart            on
AllowStoreRestart               on
MaxInstances                    10
User                            nobody
Group                           nobody
ScoreboardFile                  /var/run/proftpd.score

<Global>
 AllowOverwrite                yes
 <Limit ALL SITE_CHMOD>
   AllowAll
 </Limit>
 <Limit WRITE SITE_CHMOD>
   DenyAll
 </Limit>
</Global>


LogFormat                       default "%h %l %u %t \"%r\" %s %b"
LogFormat                       auth    "%v [%P] %h %t \"%r\" %s"

WtmpLog                 off
ExtendedLog             /var/log/proftpd/access.log WRITE,READ default
ExtendedLog             /var/log/proftpd/auth.log AUTH auth

<Anonymous ~tmp>
 User                  jdoe
 Group                 jdoe
 AccessGrantMsg                "login ok, restrictions apply."
 MaxClients                    10 "Sorry, max %m users -- try again later"
 DirFakeUser                   on jdoe
 DirFakeGroup                  on jdoe
 AnonRequirePassword     on
 <Limit WRITE SITE_CHMOD>
   DenyAll
 </Limit>
 <Directory /*>
   AllowOverwrite              off
   HideNoAccess                on
   <Limit READ>
    AllowAll
   </Limit>
 </Directory>
 <Directory Uploadz/*>
   AllowOverwrite              on
   <Limit STOR CMD MKD WRITE>
     AllowAll
   </Limit>
 </Directory>
 <Directory /mnt/emule>
  <Limit READ>
    AllowAll
  </Limit>
 </Directory>
</Anonymous>