I have bios up to date.

I dont have in bios option: Operating Mode is set to Optimizer Mode.

I re-install DIMMS and got 12 Gb memmory.

After 4-5 reboot I got the same error and only 8Gb Memmory.

I change 2 memmory dimms and now got 12Gb. I very hope that after some reboots I will still have 12Gb :)