white square in the middle of Calendar extender

there is a white square in the center of the calendar of ajaxcontroltoolkit. what is strange is this happened suddenly without any previous actions in which in all the pages of my project is the same.

here is the screenshot of the problem https://1drv.ms/u/s!ArxL2goPTnKqpv9xUsW_sDKrDIZ0Xw

<asp:TextBox ID="TxtTodate" runat="server" MaxLength="10" Placeholder="DD/MM/YYYY" onkeypress="return false" onPaste="return false" onCopy="return false" onDrag="return false" onDrop="return false" /><ajaxToolkit:CalendarExtender ID="CalExtToDate" runat="server" Format="dd/MM/yyyy" TargetControlID="TxtTodate"></ajaxToolkit:CalendarExtender><span class="highlight"></span><span class="bar"></span><label>To</label>