I'm afraid that this error code "0D0015" and the text "Reboot by hardware reset" represent the same thing, and the code itself does not point to some clearer problem source...
I'd suggest to try to boot the server with a live system from CD/DVD; either Windows or some brand of Linux, to rule out any OS or driver causes.
In case the issue still occurs, then remove every additional hardware component that isn't essentially required to boot the machine: only processor, one DIMM, keyboard (perhaps a different one?), and check again.
This would either narrow down the problem sources to these components, or to "all those other items", which you then could add again one after the other, until the problem re-occurs.