LPK9-4P PSU 12V output not working
[...] 12V line, with just 2 fans on it, just works few seconds before falling down.
  
  5V line is stable and working, powering arduino and a bunch of sensors. 
  
  Is this about load requirements?


It is impossible to be sure without a proper datasheet. However, this might be the problem, because some PC PSUs do require a minimum load on their +12V rail for correct operation. The two fans you mention as the only load on the +12V rail, would not meet the minimum load requirements for some of the PSUs which I have seen.

Without whatever minimum load a specific PSU requires, the +12V rail may fluctuate or may even rise enough to trigger over-voltage protection, and other rails (including the +5V rail) may also be out of regulation.

To test this possibility, connect a car headlight bulb (e.g. 50-60W) to the +12V rail to draw around 5A and see if the bulb remains lit. If it does remain lit, then that confirms your specific PSU requires a minimum +12V load.

Update: Of course, another possibility is that your PSU has an internal fault.