The location of the invisible "hole" is in the back of the building at checkpoint D; on the side that faces the water canal.

There are three stone steps leading to the door of that building.

When I moved across the steps, I "fell into a hole" and tried (crouching/prone/jumping/running/leaning/backwards) but could not find a way to get out of the hole.

The character is standing, not crouched or prone.

You can see below the surface.

I was not able to exit the hole so I exited the game so other plays could continue to play.