I've tried in both Firefox 43 & Chrome 45 (both on Ubuntu 15.04).
It seems reloading the page directly in the room helps, but when I leave the room and try another one, the problem remains.

Hello, I have a problem which prevents me from playing the multiplayer game. When I enter a room (no matter if it's already taken or empty), it seems there is no room map at all - I don't see room's terrain at all. The only things I can see is game's HUD, but nothing else.