My iMac (late 2015) is not responding to input devices on Activation Lock Screen

[Trambulance]

Since the MacBook Pro of my SO was stolen and we locked it. We wanted to try out how it looks now to the thief.

So I locked my iMac (late2015; Mac OS 12.4) with my iPhone.

Now when I boot it up. I can successfully enter the firmware password which was set by activation lock. (It’s apparently the same like the pin you set in the “where is” app)
Here my keyboard is working flawlessly, I can also put in the password for the wifi.

But after booting up I am stuck at that screen

No matter which input device I plug in, it just does not work, means: it does not recognise any inputs I do. It acts like I would press no buttons at all.

It wakes up though in the moment I press spacebar

Can anybody help please? Apple support was not able to help. Even after three hours on the phone.
Thank you in advance!
I really need this computer. So many deadlines.
(BTW the cracks of the screen are caused by my daughter who once used my iMac as a weight for a blanket for a cave she built for herself and it fell down. It’s several years ago and not related to my issue)

 

[Brian33]

So when the iMac is displaying the PIN-code screen you show above, your keypresses on the keyboard don't show in the PIN-code squares?

I'd try a wired USB keyboard (even a PC keyboard).

 

[Trambulance]

Already tried. Did not work unfortunately

The graphic without any visible function beneath the field to input the code is also putting me off.