You should also make sure your BIOS is up-to-date ... 1.1.4 is the VERY minimum, but I would update it to 6.2.3 (make sure to update the ESM/iDRAC firmware after the BIOS).

thnak you for the quick reply!

I got relatively new BIOS version (3.0.0)

there is no memory updates in the latets versions so i prefer no to step into it

System:         Dell(TM) PowerEdge(TM) R710
Version:        6.2.3
Release Date:   05/04/2012
_____________________

* Updated the embedded 5709 UEFI driver to version 6.2.2
* Updated the embedded 5709S PXE/iSCSI option ROM to version 7.2.3

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

System:         Dell(TM) PowerEdge(TM) R710
Version:        6.1.0
Release Date:   11/07/2011
_____________________

* Updated Intel Xeon Processor 5600 Series B1 stepping microcode (Patch ID=0x15).
* Updated BIOS ACM (Authentication Code Module) to revision 1.3 and SINIT ACM to revision 1.1.
* Corrected an issue where turbo mode was not being engaged when in Maximum Performance Mode.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

System:         Dell(TM) PowerEdge(TM) R710
Version:        6.0.7
Release Date:   08/30/2011
_____________________

* This BIOS version includes changes that can correct certain uncorrectable errors. 
* Updated Memory Reference Code. 
* Updated the iDRAC Configuration Utility.
* Enabled C state Auto Demotion capability.
* Corrected an issue where ESXi 5.0 failed to boot in UEFI boot mode.
* Removed the word "Integrated" from "Integrated Daughter Card" BIOS string.
* Added FlexAddressPlus support for uncommonly partitioned and configured CNA cards.
* Corrected an issue where Single Bit Errors were not being detected when Memory Mode was set to Advanced ECC Mode and Node Interleaving was disabled.
* Updated nested Power Profile dependencies.
* Corrected an issue where UEFI Boot Manager was displaying  multiple "EFI Fixed Disk boot device" string messages.

If anyone have another idea i will glad to get some advice

I realize the newer versions have no "documented" fixes for this type of thing, but sometimes there are undocumented fixes - I'm not saying it will fix your issue, but I've seen it too many times to discount it as a possibility.  I can also think of no other reason for it not to be available (you checked the BIOS - not just OMSA - for the ability to change to Adv ECC?) ... (assuming your memory is fully compatible) you have the correct speed of memory, a supported size DIMM (for your BIOS version), your memory is Dual-rank, and uses x4 technology - meeting all the apparent requirements for Adv ECC.  I don't see anything else in the manual, for what you posted about your config, that it could be. 

The reason i wont jump again into bios update is becuase this is production vmware host

the only way for me to update it's bios is to go into dell system tools which is high downtime for me...

of course if i will known for sure that the bios fix the issue i would have done it...

I have gone into bios after the upgrade and it would only show 64GB and Optimizer mode greyed out for change...

Also the bios prompt the memory slots a2 a3 a6 a6 are disabled during boot time... still open manage can show the helathy memory on those slots

Problem Solved!

after i contacted dell for the 3rd time i been tald that there is no support for 8X16GB in any configuration!

the only support for 16GB DIMS is by 12X16GB or 18X16GB optimizer mode only.

its very annoying that this information isn't documented at the manual and i had to spend extra $ to buy another 4 dims.

hope this post will help someone...

Thanks for the above info I just realized after seeing memory error recently.

One more thing, I read elsewhere that the maximum is actually 12x32GB, can anyone officially confirm this?

Hi, would this help?

https://dell.to/3FNQNqb

Up to 288GB (18 DIMM slots): 1GB/2GB/4GB/8GB/16GB DDR3 up to 1333MT/s

Yes. That is the official datasheet.

But there is suggestion that the maximum is actually 384gb 12x32gb.

https://cloudninjas.com/collections/poweredge-r710