Yup, you're right ! I got a another tip about the spark gap issues so I just swapped out the neckboard for the known working one on Thwocker's 6100 and the problem was gone !
Now, the CR-24 sockets are not available anywhere (well found one place, they were asking 42 euro for one, crazy but also understandable, but I wondered of that even would be a new part,
Anyway, I will have to open the CRT connector which will be hard to do without damaging it. I guess I'll have to desoldering it from the board.....