I looked further into the problem.

Using the following pinout I checked the PSU. When trickling the power supply I got all the correct readings. I guess the PSU is fine.

I went to check the motherboard and noticed two things.

1) On the backside one cap connection seems to be bad. I noticed some goo-like material  which could imply bad caps.

2) Strange!

On the motherboard just below the pile of caps there seems to be a missing component. Removed? By whom? Does anyone know what component used to be present here? Is it possible to resolder such a component back in? What do you think?