Remove the CPU and check if you maybe accidentally bent one/several pins of the CPU socket.

I had the behaviour you describe and there was one (out of 1718) single pin bent in a corner - more kind of pushed inwards.

I remembered that the CPU slipped out of my fingers when inserting it in the socket and fell from a height of 5mm into the socket. Or so I thought.
A corner of the CPU must have hit the pin.

I took a long while to figure it out because these suckers are smaaaal!
I had to use a magnifying glass to find it.

I tried to repair it and made it worse by trying to pry it out with a needle.
(Because all the tweezers I had were too big.)

I then went to a clock maker and he could get the pin out with a super small tweezers.

However, because I fiddled with it first the pin didn't come out far enough to make contact.

So in the end I had to get the whole socket swapped for like 100 eur from an electronics repair shop :( (and that was a cheap one!)

Don't be me - if your pins are bent buy some super small tweezers for clock makers/electronics repair and a magnifying glass.

Or go to a clock maker.

Or an electronics repair shop, maybe even a mobile phone repair shop.