Intel Motherboards : Fatal1ty X99M Killer/3.1 not turning off code 05

I own a Fatal1ty X99M Kill/3.1 board and have the weird problem that the board does not turn off when shutting down windows. Lights and fans stay on, display and everything else is off. I have to either switch the power supply or keep the power button pressed 10 secs to really turn it off.

It shows error code 05 during that time which, I have found out, points to a memory problem. Now, the board is rock stable even during heavy gaming sessions and I have no crashes or any problem ever. I don't get how the memory could keep the board from turning off when it shows no problems during operation.

I use corsair memory which is on the supported memory list for this board so it shouldn't be any memory incompatibility.

I also read that you should clear CMOS to try and fix it which didn't work. Another post recommended a BIOS update to fix it which also didn't work (and I am on the newest BIOS version now).

I am at my wits end and while it is not a serious issue it is starting to become annoying so I hope someone has an idea what I could try to fix this.