PCI-E x16 Not detected on Coffee Lake MoBo with Skylake-S processor installed

Greetings, everyone!

I’m having a rather weird issue.
I did CPU Microcode swapping in BIOS, to allow Skylake processor (i5-6600K) to be fitted into Coffee Lake Motherboard, and after few trials it runs successfully without any issues except for one.
I’m unable to run any device, whether it would be a Graphics Card or just a random DVB tuner I had laying around when it’s inserted into X16 Slot.
Now the motherboard is Asus GL12CP, super janky, super shitty (in terms of BIOS options) but runs well. It has 1 x16 slot and 1 x1 slot.

x1 slot works perfectly, linux is able to detect a device that’s inserted in there (DVB tuner in this case). However, inserting same thing into x16, it starts blinking (has few LED’s on it and it doesn’t do the same when it’s in x1 slot). dmesg is silent about it, mostly no errors apart from mei_me (init hw failure).
Another devices attached to PCI lane are also working (bluetooth, wifi, ethernet, you name it).

The question would be, do I need to change something in BIOS ROM related to IRQ’s? Although it doesn’t make much sense, so hoping if anybody has a better ideas.
Haven’t done any changes yet to GOP or ME, could this be the culprit?

From dmesg output, only few lines are interested (although not sure if they’re related to the problem):

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[   14.614456] mei_me 0000:00:16.0: wait hw ready failed
[ 14.614467] mei_me 0000:00:16.0: hw_start failed ret = -62
[ 14.614483] mei_me 0000:00:16.0: H_RST is set = 0x800D0D31
[ 16.630443] mei_me 0000:00:16.0: wait hw ready failed
[ 16.630454] mei_me 0000:00:16.0: hw_start failed ret = -62
[ 16.630481] mei_me 0000:00:16.0: H_RST is set = 0x800D0D31
[ 18.646437] mei_me 0000:00:16.0: wait hw ready failed
[ 18.646444] mei_me 0000:00:16.0: hw_start failed ret = -62
[ 18.646460] mei_me 0000:00:16.0: reset: reached maximal consecutive resets: disabling the device
[ 18.646463] mei_me 0000:00:16.0: reset failed ret = -19
[ 18.646465] mei_me 0000:00:16.0: link layer initialization failed.
[ 18.646468] mei_me 0000:00:16.0: init hw failure.
[ 18.646566] mei_me 0000:00:16.0: initialization failed.
 


full dmesg output : pastebin dot com/dp6NK93t

lspci output : pastebin dot com/YMx8Nvpj