DL360 G6 dual CPU endless boot

I have a HP ProLiant DL360 G6 with a Xeon X5570 and 32GB (2x16GB) 4R RAM, which works just as expected, but whenever I try to add another X5570 to the other socket (making it a dual CPU machine) it gets stuck in an endless boot loop.

I have tried the following setups:

• CPU 1 in socket 1 with 32GB RAM for socket 1= OK
• CPU 2 in socket 1 with 32GB RAM for socket 1= OK
• CPU 1 in socket 2 with 16GB RAM for socket 2= Boots, POST says "wrong socket for CPU"
• CPU 1 in socket 1 and CPU 2 in socket 2 with 32GB RAM for socket 1 = Not OK
• CPU 1 in socket 1 and CPU 2 in socket 2 with 32GB RAM for socket 1 and 32GB RAM for socket 2 = Not OK
• CPU 2 in socket 1 and CPU 1 in socket 2 with 32GB RAM for socket 1 = Not OK
• CPU 2 in socket 1 and CPU 1 in socket 2 with 32GB RAM for socket 1 and 32GB RAM for socket 2 = Not OK
• CPU 1 in socket 1 and CPU 2 in socket 2 with 16GB RAM for socket 1 and 16GB RAM for socket 2 = Not OK

I have checked the pins on the board for CPU socket 2 and they all look good / the same as for socket 1.

The endless boot loop works as following:'

• Power on the server
• Fans starts up at max
• Fans slows down after a couple of seconds
• I hear a ticking sound about one tick per second and the fans start to gradually speed up
• After a while at max speed, the fans suddenly goes very slow again and I can hear the ticking sound again while the fans gradually speeds up again

POST is blank/says nothing during this boot-process.

When I install the second CPU I also install another fan-kit (making it 4 of them installed) as instructed.

Versions and specs:

• SPP 2017.04.0 (last service pack to support G6)
• iLO2 v 2.32 (2018-02-06)
• System ROM P64 2015-08-16
• 2x 750W PSU's

Questions:

• Does the ProLiant support a single CPU in socket 2 and none in socket 1? (testing purpose)
• Is there something else I can do to error search this issue?

Thanks in advance.