ERROR: reading the cookie off

[vehl_g1]

My cacti when i want to save graph show me this error
RRDTool Command:

/usr/bin/rrdtool graph - \
--imgformat=PNG \
--start=-86400 \
--end=-300 \
--title="LevelOne ADSL - ADSL" \
--base=1000 \
--height=120 \
--width=500 \
--alt-autoscale-max \
--vertical-label="0" \
--font TITLE:12: \
--font AXIS:8: \
--font LEGEND:10: \
--font UNIT:8: \
DEF:a="/var/www/htdocs/cacti/rra/levelone_adsl_snmp_oid_8.rrd":snmp_oid:AVERAGE \
AREA:a#F5F800FF:"soheil" \
GPRINTLAST:"Current\:%8.2lf %s" \
GPRINTAVERAGE:"Average\:%8.2lf %s" \
GPRINTMAX:"Maximum\:%8.2lf %s\n"

RRDTool Says:

ERROR: reading the cookie off /var/www/htdocs/cacti/rra/levelone_adsl_snmp_oid_8.rrd faild

[gandalf]

Please run a "file <rrd file>" command and post results as well as the output of "rrdtool --version"
Reinhard