OK, it seems the E2X00 Xeons don't support any other ECC types: Is Registered ECC Memory Compatible with Intel® Xeon®...
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Description Can I install the Intel® Xeon® E-2224 Processor and Registered ECC DIMM (RDIMM) memory on a server board?
Resolution Only Unregistered ECC DIMM (UDIMM) memory is supported with all Intel® Xeon® E-2100 Processors (formerly codenamed Coffee Lake) and Intel Xeon E-2200 Processors (formerly Coffee Lake Refresh).```
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Description Can I install the Intel® Xeon® E-2224 Processor and Registered ECC DIMM (RDIMM) memory on a server board?
Resolution Only Unregistered ECC DIMM (UDIMM) memory is supported with all Intel® Xeon® E-2100 Processors (formerly codenamed Coffee Lake) and Intel Xeon E-2200 Processors (formerly Coffee Lake Refresh).```