This might relate. I have not confirmed this but I believe the memory controller on the xw8400 is the same as on the xw6400.
I noted for a prior project that HP officially supported specific 8GB sticks in the xw8600, and that the xw6600 and xw8600 memory controllers were the same. That gave me the idea that perhaps, though not documented, those same sticks for the xw8600 could work properly in the xw6600.
I got some of those HP 8GB sticks and confirmed that they would run in my xw6600 builds just fine. But, they would not run in my xw6400 builds.... same sticks, same perfectly functioning workstations if they had HP 4GB sticks in place.
I believe you are running into a memory controller issue on that older generation xw8400 workstation..... the earlier (6400/8400) and later (6600/8600) generation workstation familes I note above have different memory controller chipsets.