Hi,
In my map, a wall just appeared. But it only appears when playing the map, there is no sign of it in hammer. Also, i can walk throug that wall(it seems to be 1 unit thick). Any ideas, how to remove it?

Could you post your .vmf? is it an unvisible Wall? If yes, maybe it is the hitbox of something

This usually happens when you have been editing a brush and you either make it less than 1 unit thick or you cut a portion that is less than 1 unit thick. I recommend you go to the area where the "wall" is when you are running HAMMER. Do a box-select around the area. This will select the "invisible" object. Then just delete the object. It is a glitch and has no real effect other than to cause a fake wall in-game.