I already tried to set up another the MAX_MENU-variable in the scetch instead of the MENWIZ.h. So it had no effect. 
Now when I change it to 25 it works (but I haven't set up half of the variables I want to change). When I set it to 30 there occur errors in the menu. I hope 25 is enough to set up all menus.
I need menus to set up time, 2x on-times for the light, 2x off-times for the light, moon-light duration, temperature and fan-offsets, CO2 on and off, 2x on and off for the dynamic pump, 2x time and duration for the fertilizer pumps as well as activation variables for some of the options. Seems to be some extensive menu making ahead. 
Thank you for the help!