Sys.WebForms.PageRequestManagerParserErrorException: The message received from the server could not be parsed

I fixed this issue. As I'm using UpdatePanel, I added below code in the Page_Load event of the page and it worked for me:

protected void Page_Load(object sender, EventArgs e) {
  ScriptManager scriptManager = ScriptManager.GetCurrent(this.Page);
  scriptManager.RegisterPostBackControl(this.btnExcelExport);
  //Further code goes here....
}

In my case, the problem was caused by some Response.Write commands at Master Page of the website (code behind). They were there only for debugging purposes (that's not the best way, I know)...

I added the control to the Triggers tag in the update panel:

    </ContentTemplate>
    <Triggers>
        <asp:PostBackTrigger ControlID="exportLinkButton" />
    </Triggers>
</asp:UpdatePanel>

This way the exportLinkButton will trigger the UpdatePanel to update.
More info here.