I haven't had this problem, but I have seen two different people in the last week having it, with the exact same door. One of them actually beat down the door with a pickaxe, and even once it was down they still couldn't go in, like there was an invisible wall. They also tried restarting the game and re-loading from a previous save and nothing's doing it.
Okay, so, someone just did that and it still didn't work, but then they beat down the door with the pickaxe, and were able to crouch under the invisible wall. So maybe the OP can try that, if the previous advice doesn't work for them.