The error occurs in  the function "Sys$UI$_UpdateProgress$_startRequest()"
It fails on : " this.get_element().style.display = 'block';"

 function Sys$UI$_UpdateProgress$_startRequest() {
        if (this._pageRequestManager.get_isInAsyncPostBack()) {
            if (this._dynamicLayout) this.get_element().style.display = 'block';
            else this.get_element().style.visibility = 'visible';
        }
        this._timerCookie = null;
    }