Best measure is 16 units,might be 15.
In a turnine stair it is better to keep the steplength more than 16, aproxemate 24. 

Look up #define STEPSIZE here

https://github.com/id-Software/Quake/blob/master/WinQuake/sv_move.c

The height for climbing a step is applied equally for both players and monsters. But one of the differences is how smoothly the movement is applied. A player moving at 360 units per second on a modern computer would have that movement split up into 72 segments of length 5. Each movement can climb one step, so the player can comfortably manage many steps close together.

In contrast, a monster moving forwards at 360 units per second would move in just 10 segments of 36 units each. And the movement is all or nothing - if the monster would need to climb TWO steps to reach the end point, it will collide with the second step and the entire movement is aborted.

So putting more horizontal spaces between the steps will help a lot. Monsters will also never go off a ledge with drop a of more than 1 step using walkmove or movetogoal - you have to give them velocity movement if you want them to really fall. Be careful, it's easy to turn see that feature turn into an exploit as your monsters can now be lured into pit traps and completely bypassed. 

Another alternative is to split up the navigation into small segments within a frame, for example replacing movetogoal(20) with movetogoal(10); movetogoal(10); It's not always a 1 to 1 equivalence, like with lots of Quake AI it's a thing to experiment with and tune. 

This error means that you've forgotten to set something, but the error message doesn't tell you which something it is. Some possibilities are

th_stand - the AI for an idle monster
th_run - the AI for a newly alerted monster
th_pain - the AI for a pain animation
th_die - the AI for a death animation
think - the next function to run after a timer.

To try and diagnose, add this line to the START of your monster spawn function

th_stand = SUB_Null;

If that fixes the error, you know that you've forgotten th_stand - and NOW you need to fix that by giving the monster the AI, this is a diagnostic tool only. If that doesn't help, add another line

th_run = SUB_Null;

Repeat until adding a line avoids the crash, then develop a fix. 

soo... you're saying it cannot be changed via mod? it's engine based?