Tim-B,  There is a couple things to verify. What is the processor installed, as well as the riser version in the server? With the error you are getting it is normally due to a processor incompatibility, which in the R810 the 16GB dimms are not supported in the case that a Nehalem processor is installed. Also, the Low Voltage dimms will only be supported in the R810 if the memory riser has the roman numeral II on it.  Let me know if this is what you are seeing. 
My R810 is currently fitted with 64GB (16x 4GB) sticks of DDR3-8500R and we were wanting to bump this up to 128GB. We've got four E7540 Xeons fitted.We purchased four IBM DDR3-12800R ECC ram (2Rx4, 1.5V, IBM part number is 00D4968) which I thought should be OK, if overspec'd. Crucial sell some similar RAM (edge.crucial.com/.../mpartspecs.aspx) which seems to be identically spec'd, aside from being 1.35V - and that page states guaranteed compatible with R810. My error is: unsupported: DIMM A1 Lockstep pair DIMM A1 & A2 disabled. Please replace the DIMM or remove the lockstep pair. So why isn't my DIMM recognised? Is it safe to boot anyway (I can't remember if it gave me the option, I reverted back to the original 4GB sticks in order to install a new OS.) My RAM configuration with the mismatched RAM was: A1-A2: 16GB IBM sticks (2 slots)B1-B8: 4GB 8500R sticks (8 slots fully populated)C1-C8: 4GB 8500R sticks (8 slots fully populated)D1-D2: 16GB IBM sticks (2 slots) On boot I get the error for both A1 and for D1, so I don't believe it was a RAM seating error since it always applied to all slots with the 16GB sticks. 
R810 unrecognised DDR3-12800 DIMM