If I'm right, I believe it means that you are already in game, but for an unknown reason you see yourself still in the waiting room and the game doesn't know why you are trying to join a game you are already in.
If this error message pops out, simply refresh the page and you should be fine.
I was playing Taklons. The situation was that I had passed, but I was still willing to charge. However, the game automatically declined the power.
The power was 1/0/3(I/II/III) with PI when I passed, where the brainstone was at the first pool.
I did check the settings, I'm sure that neither "auto charge" was at "decline if any cost", nor "auto brainstone" option was checked. I wonder if it was a bug, or it was just me messing with something?