I tried to reproduce the issue but to no avail. I am attaching sample project based on your scenario to find the difference with your project.
However, I think that you are disabling the panel control with the Visible=false property. And because of this the getElementByID function is returning null and the request is broken. This is true because when Visible property is set to false the control is not rendered on the page at all and because of that the element could not be found.
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