To follow up, I can confirm that all the DIMMS work - just not all at once. Any suggestions?

Try to put new memory in pc without old memory.

Thanks @uco73 , installing the memory in the 5820 XL on it's own works. If I then add the old memory the system fails to boot and I get the 3 beeps, 2 orange lights, 4 white lights indicating RAM failure.

If I remove all and reinstate the original memory it boots successfully again. It also fails if I use 3 DIMMS (2 x original plus 1 new).

The purchased memory type is exactly the same as the preinstalled RAM (16GB 2Rx8 PC4 2666V), with the same P/N.

Can the 5820 XL use 4 x 16GB 2Rx8 PC4 2666V ECC DIMMS?

I have 5820 XL too. Inside pc is 8x32GB, W-2195 and RTX 3080 Ti. Everything working flawless.

Mine is using a W-2155. I would purchase additional 16GB DIMMS to fill the banks but don't want the expense if the system is not compatible,

@n3wu5er​ I have Samsung memory DDR4 ECC registered 2400MT/s.

@n3wu5er​  Is the new RAM also by the same mfr.?  Some Dell's and some PC's can be picky about that.

Yours is far from the 1st case whereas a Precision won't take added RAM.  Difficult to diagnose.

Hi @bradthetechnut , yes same mfr, Sam mem P/N. Think I am going just have to purchase a replacement set of dimms.