So apparently after reading here I'm not the only one seeing this. The "error" message lasts for the same amount of time the AD is supposed to play. Hitting "refresh" no longer works for me either, the stupid "error" message just comes back followed by a "Playback ID" number.
Is Google adapting now to Firefox AdBlockPlus? What's being done about this issue? It's basically rendered AdBlockPlus ineffective at the moment.