Saving a custom configuration section results in a System.UnauthorizedAccessException error

I have the following problem trying to use a custom configuration section in my app.config. I use a custom configuration section to track the selection of folders that I want to create using my program:

<CustomConfigSection>
    <BackupLocations>
        <clear />
        <add path="C:\Users\Marcel\Documents\" />
    </BackupLocations>
</CustomConfigSection>

Now, when I save the configuration file, I get this exception:

System.Configuration.ConfigurationErrorsException: an error occurred loading the configuration file: access to the path C: \ Program Files (x86) \ Backup solutions \ uqhuxi1j.tmp deprived. (C: \ Program Files (x86) \ Backup Solutions \ BS.exe.Config) ---> System.UnauthorizedAccessException

Code I use:

Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
CustomConfigSection section = (CustomConfigSection)config.GetSection("CustomConfigSection");

section.BackupLocations.Add(element);

section.SectionInformation.ForceSave = true;
config.Save(ConfigurationSaveMode.Full);

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AppData, Custom Config ApplicationFolder/exe.config? AppData?

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