Hi Tassos
Ive seen this problem before on the Etv the problem is in the hydraulic control valve in the reach carriage the lift spool will be stuck you can try and strip and clen it out you do this by removing the solenoid valves but by experience i know you will need to replace the complete control valve. It is critical on the Etv that the hydraulic oil is changed every 2000 hours. You will need to call in your local jungheinrich tech to teach in the valve block it will possibly work but you need to plug in a laptop to input the number on the valve block into the controller.

Hope this helps

I'm not too familiar with these machines but try disconnecting the plugs from the lift section solenoid valves. If problem still exists it is likely internal in valve block. If this stops it then either a back feed to solenoid a or faulty lever pot.
Hopefully someone with more experience of these machines can help further.
Regards, Simon