I had the same problem. It occurs if you frequently mount snapshots. This error occurs around the time you have 512 old entries. You can remove them one at a time as mentioned above. Here is a script to remove all the stale entries:
esxcli storage core device detached list|grep -E "^naa|^eui"|awk '{print $1}'|while read i;do echo "Removing device \"$i\" from the detached list ...";esxcli storage core device detached remove -d "$i";sleep 1;done