The only electrical difference between the older 5pin DIN keyboards and the newer 6pin DIN keyboards is the plug. Voltages and signals are identical (except on 8088 machines).
Is it possible the adaptor you're using isn't meant to connect a keyboard but rather a MIDI device or something else?

That's bloody weird.

Yes, try another adapter and maybe test if they have the exact same wiring or not - if you own a multimeter or line tester of sorts.

But before you risk another keyboard, try at least to test through the AT DIN port on your motherboard, unlikely, but maybe there is something really rotten about it.
Like the wrong voltage or voltage at the wrong spot. Look up the pinout somewhere, shouldn't be hard to find.

If you don't own a multimeter - the basic ones are dirt cheap 😉

Yeah what they ^ said ! Maybe the PS2-AT adapter is wired wrong and putting the +5v on the CLK or DATA lines or swapped the GND and +5....

If it's adapter from aliexpress it might be wired wrong. Unless it works fine on other boards, in this case I'd check around AT connector of this board for shorts. Maybe someone did some repair on it, changed the fuse, or whatever it has (my QDI Titanium IB+ has transistor to control voltage on AT / PS/2 ports) and soldered something wrong.

Few months ago I was looking to buy few on aliexpress but comments made me reconsider. People had exactly these types of problems and when they checked continuity it turned out to be wired wrong. In the end I made an adapter myself.

Solo761 wrote on 2022-03-31, 06:02:

If it's adapter from aliexpress it might be wired wrong. Unless it works fine on other boards, in this case I'd check around AT connector of this board for shorts. Maybe someone did some repair on it, changed the fuse, or whatever it has (my QDI Titanium IB+ has transistor to control voltage on AT / PS/2 ports) and soldered something wrong.

Few months ago I was looking to buy few on aliexpress but comments made me reconsider. People had exactly these types of problems and when they checked continuity it turned out to be wired wrong. In the end I made an adapter myself.

It's just how the world works.
Old friend of mine is an audio engineer. His professional estimate: About half of these "3.5 mm (1/8") mini audio jack to 2 x RCA" cables out there swap the stereo channel.

Also, many setup programs of old DOS games had options to reverse stereo because many sound cards would mess that up, too.

Just rare that it does any real damage.

Then again, neither the 5x DIN nor the 6x mini-DIN (PS/2) are dedicated Keyboard connectors.
So, there's not one obvious "correct" way to wire an adapter. Maybe Doornkaat was right and it merely was meant for another purpose.

I was able to check the cable today with a multimeter and the pins are indeed wrong. I used this page as a reference: https://pc-control.co.uk/keyboard_connector.htm

Ground and 5V seems swapped, I'm not sure why, maybe they used male to male pinout, or like some of you said, it's for another purpose. Cable doesn't have any marking on it other than "Cable Made In China". Anyway, thanks for the answers. I guess I'll test any converter cable for retro purposes before using from now on 😀