I have moments in my script where this crashes:

temp.x = screen_size.x;

Is there times when screen_size is invalid/removed, like during loading a level?

Temp is valid, I checked that.

And temp is a VECTOR?

Yes. This always happens to me... every time I post a bug, people ask me about temp ^^.

That's why I mentioned it up above.

The line that really crashes is:
darkenPan.size_x = screen_size.x;
But to try and see if darkenPan was the issue, I replaced it with this:
sys_marker("ds2");
temp.x = screen_size.x;
sys_marker("d02");

And then I got a crash in ds2.

Engine variables like screen_size can not suddenly disappear or something like that.

But you can, of course, make it disappear, f.i. with "screen_size = NULL;" or by writing at wrong positions into the variables memory. This would then cause such an error.