If rrd files were created e.g. with root ownership, a poller running as cactiuser will not be able to update those files
cd /var/www/html/cacti/rra ls -l localhost* -rw-r--r-- 1 root root 463824 May 31 12:40 localhost_load_1min_5.rrd -rw-r--r-- 1 cactiuser cactiuser 155584 Jun 1 17:10 localhost_mem_buffers_3.rrd -rw-r--r-- 1 cactiuser cactiuser 155584 Jun 1 17:10 localhost_mem_swap_4.rrd -rw-r--r-- 1 cactiuser cactiuser 155584 Jun 1 17:10 localhost_proc_7.rrd -rw-r--r-- 1 cactiuser cactiuser 155584 Jun 1 17:10 localhost_users_6.rrd
Run the following command to cure this problem
chown cactiuser:cactiuser *.rrd