HP ProLiant ML350 G6 powerup issue

My setup: HP ProLiant ML350 G6 (2x
PSU/redundant)

I present my issue in a few
steps:

1. Power
   cables disconnected (no power coming to
   server)


2. Re-connect
   cables


3. Press "Power button" -> I can see green lights,
   I can hear fans spinning (it's actually quite loud) and server will power up
   fine


4. Now if I power down server (but keep power cables on
   the back as they are)


5. Press "Power button" -> I can
   see green lights, but there is no fans sound coming out and server will not power up at
   all. All seems fine, correct lights, but no beeps and no video output on the screen. It
   just stays black.

In
order to power up the server again. I need to disconnect both power cables from PSUs (on
the back) and then re-connect them back and upon pressing the power button we are back
at step no.3 and all is fine, until next
restart.

I have tried following:

• connect one power cable
  from mains and the other from APC -> same
  result


• connect both power cables from APC -> same
  result


• disconnect APC "data" cable from PC -> same
  result

Am I missing
something here? This is very annoying when we have to remotely restart server for weekly
(un)happiness of Windows
Updates.

Looked at all BIOS settings
but there is nothing really there that could point into this issues. There are some
Power options (aka Economy settings etc) but all is left as default.

Any ideas would be welcomed.
Thanks.