Some Hysters use a vacuum dashpot on the carb's throttle lever to speed up the engine when it's about to stall. As the engine stalls, the dashpot loses vacuum and extends its actuator which hits a lever on the throttle and "catches" the engine before it shuts off from the RMPs dropping to low.

If so equipped, make sure it's adjusted properly or is working at all.

Thank you Sir for your answers.
We were thinking about orbitrol valve but we didnt wanned to do that because its very expensive.
Today we visited another customer and he had the same problem with his hyster even thou it had only 900h.

you'll want to check all your pressures first
Hydraulic Pump
Type Gear
Priority Flow 10 liter/min (2.64 gal/min)

Relief Pressures
Steering (at Pump) 10.6 to 11.7 MPa (1540 to 1700 psi)
Primary (at Control Valve) 21.37 plus or minus 0.5 MPa (3100 plus or minus 75 psi)
Secondary (at Control Valve) 15.5 plus or minus 0.5 MPa (2250 plus or minus 75 psi)

if all those are at correct settings then you might need to check for something binding at the wheels or just by some slight chance you have a flaky priority valve or relief valve or even possibly a flaky orbitrol valve.

engine stalls when turning wheels when forklift is not moving.

Does the engine stall only when turning the steering or only when the steering dead heads?