OK so here's an update after a little debugging. I unplugged the connector to the RAMPS board and the power supply and everything else (fans lights) powered up fine. I turned off the supply, plugged back in the RAMPS and the LCD once again flickered on and then the power supply stopped. (as a side note the supply needed some time to recover before I was able to power up again with the RAMPS disconnected.

So now I have ruled out a damaged supply and the 12V wiring to the fans and LEDs. I must have a short or other issue with my RAMPS board.

If anything jumps out to anyone of something to check first please let me know! I already checked the screw connections on the RAMPS board and everything seems tight and fine.