I have exactly the same problem. Tried those floor tiles in a whole lot of variations but that stupid door will not open.
This game sucks when it comes to locked chests and doors. You can look yourself silly trying to find keys, pressure plates and the likes sometimes and can never find it.
I think they tried to find ways to make the game longer to save on content, so they tried to be clever and put these stupid stupid things in there! I am getting really angry sometimes!