Can't override XML config settings:ConfigurationErrorsException: Invalid key value

I am trying to build a console application, where the user can decide, wether to enable logging or not. For that I got a app.config file in XML format. The Code can succesfully read the current state, but when I am trying to override the "protocol" property, I get the given exception in Line 27

My protocol class:

    using System;    using System.Collections.Generic;    using System.Text;    using System.Configuration;    using System.Collections.Specialized;    namespace Bruch    {        class Protokoll        {            public Protokoll()            {                string protokollConfigStatus = ConfigurationManager.AppSettings.Get("protokoll");                if (protokollConfigStatus == "unknown")                {                    SetzeEinstellung("protokoll", "test");                };            }            public static void SetzeEinstellung(string key, string value)            {                Configuration configuration =                    ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);                configuration.AppSettings.Settings[key].Value = value;****************configuration.Save(ConfigurationSaveMode.Full, true);     //this line                ConfigurationManager.RefreshSection("appSettings");            }        }    }

my config file:

<?xml version="1.0" encoding="utf-8" ?><configuration><appSettings><add key="protokoll" value="unknown" /></appSettings></configuration>

The error message:

>System.Configuration.ConfigurationErrorsException: "An error occurred executing the configuration >section handler for configProtectedData.">>inner exception:>ConfigurationErrorsException: Invalid key value.