Most likely, the Maximum speed for the Z axis is too high: try 10 mm/s.
The Maximum acceleration may also be too high: try 100 mm/s².
Set the Keypad jumpoff speed and Keypad acceleration for the Z axis to similar values.
The 6445 manual describes several autofocus settings that surely make a difference, but I do not understand what the various choices mean; they seem to depend on the exact hardware setup. Trying various combinations may be the only way to find one that lets LightBurn control the focus.