I don't know, but I've seen old keyboards cause weird problems. About a month ago I was testing an Asus Slot-A board from ebay, and I got disappointed thinking it didn't work. Turned out it just hated both of the old keyboards I had used. It wouldn't POST if they were plugged in. With a modern keyboard it worked fine.

Sometimes I used to get a keyboard error, in which the computer wouldn't boot if there wasn't any keyboard plugged in...or if there was one plugged in and it just didn't detect it, like if it didn't recognize USB keyboards.

Other times I have to unplug it and plug it back in for the computer to realize that there's a keyboard stuck to it.

Is it possible for a keyboard to cause halt errors??
Now i started getting halt errors.

I've had similar errors when using a long keyboard extension lead. I just stopped using that lead and the errors went away...

So that means there is a delay somewhere?
Could it be caused by dirty contacts? Or maybe old capacitors?

I put it down to signal degradation introduced by the extension lead, rather than 'delay'. Now, this could've been from poor contacts/ poor cables, and the signalling may have already been degraded from old electronics - the extension lead just made it worse...

The extender i use for PS2 is 10cm in length.
I tried also using an different cable with PS2 only and the same thing happened.

Could this mean the other, old electronics?
Would replacing the old capacitors and oscillator do any good?

Have you tried an AT keyboard without any converters/ extenders?

Before doing anything on the motherboard side, I'd try that first. It could also be the switches in your keyboard failing/ dirty...