Ok Found your problem, you are passing Int64 as parameter which should be string otherwise so when changing it to below I get success message :
 [System.Web.Services.WebMethod()]
 public static string btnPostReminder(string TicketId, string remindertext, string reminderon)
    {
        return " successfully";
    }
Also your data should look like below:
 data: '{"TicketId":"' + re + '","remindertext":"' + remindertext + '","reminderon":"' + res + '"}',
Pass as string for parameter and to TryParse to see if it is of type int.
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solved jquery ajax post method giving internal server error [closed]