I just tried again and it's working now!
Looking at the "System Logs", I see the following line for about 60 times spanned over about 2 minutes.
ntop: **WARNING** RRD: rrd_update(/var/db/ntop/rrd/interfaces/re1/throughput.rrd) error: illegal attempt to update using time 1337163807 when last update time is 1337163923 (minimum one second step)
The following line also appears for about 15 times in the same log:
php: : The command '/usr/bin/nice -n20 /usr/local/bin/rrdtool update /var/db/rrd/system-memory.rrd N:U:U:U:U:U' returned exit code '1', the output was 'ERROR: illegal attempt to update using time 1337163734 when last update time is 1337163897 (minimum one second step)'
Anyone has any idea whether these 2 issues could be the cause of the problem?