A GUI control had this in it.
PHP Code:
this.x = 9;
this.horizsizing = "left";
When it should be:
PHP Code:
this.x = 9;
this.horizsizing = "right";
Which also fixes the problem described in this thread.
Also, the screenwidth limit changes when you relog, plus this error also causes the screen to not center on the player properly, making it possible to walk outside the screen without leaving the level bounds.