Checked it out today, and yes it looks like the segments have separated from the discs and jamming it.
I could see some forklifts, have made it so it's possible to remove/change them without taking the whole gearbox out.
But I'm probably not thst lucky on this one.
I'm still waiting for the service manual, so I will know for sure when this arrives.

Thanks all for your inputs :)

I have seen many drive disc that have separated from the steel plate and jam both clutch packs not unusual not sure if it gets hot or wrong oil or just age?

No, not as far as I remeber.
But as I wrote earlier, when I loosened one of the clutchpack, the forklift drove forward no problem.
So it is just that both clutchpack are always engaged somehow.

Is there any bull gears on drive wheels sometimes a ball bearing gets out of its cage and gets stuck

Yes, god tip, easier with air than rigging up with oil pressure pump :)
Thanks.

with the valve body off you might can use an air wand with a rubber tip and put pressure to the ports on the transmission that go to the clutches and see how they react. I've done this before (old school method) on older trucks that are strictly hydraulic. Yours sounds like this to me. Just do not apply too much pressure, just enough to get the clutch pack to react and see if they use pressure to release or apply. You should get movement in either case.
Having the hyd diagram for the transmission would be needed to know which ports do what.

It is not as far as it may look like, only took of the valveblock on top to get to this.
If I have to take out both clutch packs, then I need to take the gearbox out and split it open. And that is a job I preffere in the longest not to do :)

There is actually no solenoids for this, only mechanical.

Of course some channels could be plugged/chocked, but I was thinking if there is no oil pressure, both clutch should be in neutral?
Or is it opposite, that it needs pressure to release both clutch packs?
If that is the case, I will have to check the valve block more thorough.
I've ordered the service manual, so hpefully there will be some schematics there, and maybe also some pressure on various test points.

If it is opposite, like it needs oil pressure to free both clutchpacks, then I could try to put pressure where the inching valve is connected?

since you're into it this far you might as well pull the clutch packs and dissassemble them and see if there are any broken swash plates between the friction discs or any other problem that would cause a full lockup of the fwd/rev. I have seen cat trucks do this before and it was usually a broken swash plate.
The chances of it being the wrong clutch discs is not likely.

The only other explanation i can think of could be a solenoid control valve malfunction, but you should have checked this before diving into the internals of the transmission.

Another thing, when I remove the valve block on top, should not both clutchpacks be free?
And it should need oil pressure to activate?

But I can't see how both clutchpacks(forward and revers) could be jammed at the same time??
If one where jammed, I could understand it is possible, but both seems a bit strange!?

When I write I loosened one of the clutchpack, then I just removed the locking/retaining ring, tha keeps the clutch disc inside the basket. so they became loose.
And then it would go forward all the time, but still it will not go in neutral...
I also been thinking if someone could have changed the clutch discs and put in wrong type, so that they are too thick..
But the guy I bought it from, said it suddenly happened by itself.

Any ideas?

Here is a picture.
https://imgur.com/a/W0DKbuj