Mysterious invisible block

[Damascus]

Woo, my first post in months! I'm almost finished my map, but I'm having a very curious problem with this particular staircase:



At the very top of the staircase, there is something blocking the player (and enemies) from passing, just beyond the stair side of the doorway. Pushing against it allows for some sort of give, and then remaining motionless pushes you back. I've had no problem with any other staircase in the map, and there is nothing on the Level Editor to show that there is something blocking the way.

I assume there must be some sort of glitch in the meshes of any of the surrounding static objects, but I have no way of telling what it is or how to fix it. Has anyone else had similar problems or know how to fix it?