X540-T2 causes boot failure if third DIMM slot is used

I am trying to install Intel X540-T2 (10 Gbps ethernet cards, Dell Part : 540-BBHD) in some Dell Optiplex 7040 (i5) machines.

The machines fail to boot unless I remove RAM from the third and fourth DIMM slots on the Optiplex's motherboard (reducing 32 to 16 GB RAM). I assume this is a memory mapping issue.

Is anyone aware of a BIOS issue causing this or have any suggestion on how to fix/work around this problem?