[mrtg] MRTG resets all graphs daily, why?

[Adam Williams]

This is what I'm getting in emails from cron about mrtg.  It says rateup 
can't rename log files.  Which is strange, mrtg is being ran as root in 
cron, so there should be no problems with file permissions, and there's 
plenty of free disk space from my last email (18% free).  Any ideas?

Date: Fri,  5 Jun 2009 08:40:07 -0500 (CDT)
From: Cron Daemon <root at missioncontrol.squeezer.net>
To: root at missioncontrol.squeezer.net
Subject: Cron <root at missioncontrol> env LANG=C /usr/bin/mrtg
    /etc/mrtg/mrtg.cfg --logging /var/log/mrtg.log

2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup could not read the
primary log file for localhost_68.209.223.242
2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup Can't remove
localhost_68.209.223.242.old updating log file
2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup Can't rename
localhost_68.209.223.242.tmp to localhost_68.209.223.242.log updating log
file
2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup could not read the
primary log file for localhost_10.1.3.1
2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup Can't rename
localhost_10.1.3.1.log to localhost_10.1.3.1.old updating log file
2009-06-05 08:40:07, Rateup WARNING: /usr/bin/rateup Can't rename
localhost_10.1.3.1.tmp to localhost_10.1.3.1.log updating log file

and all that /var/log/mrtg.log contains are entries like this:

2009-06-05 08:40:06 -- Started mrtg with config '/etc/mrtg/mrtg.cfg'