The riser you are referring to, riser 2, is part number 6R1H1. It is the riser that requires both processors to be populated.
Let me know if this helps.
Thank you very much for your reply, but I could not find anything with that part number.
Perhaps it's 330-BBCM?
The problem is that after installing the second CPU, there is an error message on the front panel that says "System board Riser cable or interconnect failure. Check connection".