zabbix server complains in the logfile :
[A Test Host:vm.memory.size[used]] error: Invalid first parameter in function [last(vm.memory.size[total])]

calculated item formula is :

last(vm.memory.size[total])-last(vm.memory.size[free])

both items exist and return proper data (for the total memory, last value is 2105892864 and this item is updated every 30 minutes)

probably should also be checked whether this issue exists with 1.8 branch.