I had the same issue on my vcenter.
I think the problem comes out to the people who have configured 8GB of memory for the Vcenter appliance.
If you log into vcenter shell and digit "free -m" you probably see that used memory is near the total, and this cause the alarm.
I've increased the appliance VM memory to 16gb (maybe 10-12gb are enough) and the error has not come out yet.
I hope that this tweak resolve your problems.