Yes, I'm using c_move. I have the eflags FAT and NARROW set so I can change the collision box. Increasing the width and length of it doesn't seem to help, since once the player gets inside a block, nothing stops him. Collision with that block doesn't work until he gets out himself. No, I did not use c_updatehull, I manually set the max_x and min_x.

The red oval in my drawing is representing the collision ellipsoid of the player, and I have it set to be as big as the player model, except for the bottom, which is a bit shorter than the actual model. Making it the proper length doesn't fix the issue, though.

max_x is 8, 8, 45
min_x is -8, 8, -20

I think one thing causing the issue is the fact that the collision box is an ellipsoid. Since there's an opening, some of the ellipsoid is able to go through before colliding with the upper wall. Of course, this isn't the main reason, because I can do this without jumping, which simply results in the player getting slightly closer, and gliding horizontally getting all jittery.